We’re going to use Stan. Set some settings:

library(rstan)
library(rstanmulticore)
rstan_options(auto_write = TRUE)
options(mc.cores = parallel::detectCores())

First, connect to the Wordbank database and pull out the English WS and WG data.

wordbank <- connect_to_wordbank()

admins <- get_administration_data() %>%
  filter(language == "English")

items <- get_item_data() %>%
  filter(language == "English", type == "word") %>%
  select(item_id, form, category, lexical_category, item) %>%
  mutate(num_item_id = as.numeric(substr(item_id, 6, nchar(item_id))))

get_form_data <- function(input_language, input_form) {
  form_items <- filter(items, form == input_form)
  get_instrument_data(input_language, input_form, 
                      form_items$item_id, iteminfo=form_items) %>%
    mutate(produces = value == "produces",
           understands = value == "understands" | value == "produces") %>%
    select(-value) %>%
    gather(measure, value, produces, understands) %>%
    left_join(select(admins, data_id, age)) %>%
    filter(!is.na(age)) %>%
    mutate(form = input_form)
}

ws <- get_form_data("English", "WS") %>%
  filter(measure == "produces")

Now aggregate by item.

items <- ws %>%
  group_by(item, age) %>%
  summarise(produces = mean(value, na.rm=TRUE))
            
write_csv(items, "ws.csv")

Now arrange.

ordered <- items %>%
  group_by() %>%
  arrange(age, desc(produces)) %>%
  group_by(age) %>%
  mutate(index = 1:n()) 

Plot this with a glm sinusoid like in the Mayor & Plunkett (2011) paper.

qplot(index, produces, facets= ~age, 
      geom = "line",
      data = ordered) + 
  ylim(c(0,1)) + 
  geom_smooth(method = "glm", 
              family = "binomial",
              formula = y ~ x) 

Try a polynomial fit.

qplot(index, produces, facets= ~age, 
      geom = "line",
      data = ordered) + 
  ylim(c(0,1)) + 
  geom_smooth(method = "lm", 
              family = "binomial",
              color = "red",
              formula = y ~ poly(x, 3)) 

Doesn’t work that well for the younger ages, though looks fine later.

Note age interactions in the stats.

mp.glm <- glm(produces ~ index * age, family = "binomial", data = ordered)
summary(mp.glm)
## 
## Call:
## glm(formula = produces ~ index * age, family = "binomial", data = ordered)
## 
## Deviance Residuals: 
##      Min        1Q    Median        3Q       Max  
## -0.51045  -0.09580  -0.01266   0.07168   1.16561  
## 
## Coefficients:
##               Estimate Std. Error z value Pr(>|z|)    
## (Intercept) -4.695e+00  2.839e-01 -16.538  < 2e-16 ***
## index       -7.769e-03  8.480e-04  -9.161  < 2e-16 ***
## age          2.640e-01  1.251e-02  21.100  < 2e-16 ***
## index:age    9.173e-05  3.489e-05   2.629  0.00855 ** 
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## (Dispersion parameter for binomial family taken to be 1)
## 
##     Null deviance: 4140.24  on 10199  degrees of freedom
## Residual deviance:  175.65  on 10196  degrees of freedom
## AIC: 5984.7
## 
## Number of Fisher Scoring iterations: 5

Further thoughts on Mayor & Plunkett model

The Mayor & Plunkett (2011) model has two corrections. The first uses the parametric form of the logistic to fill in low-frequency words that are not on the CDI, the second uses the difference between diary study counts and CDI counts to find a multiplier for higher-frequency words that are omitted.

I think there’s a bit of a conceptual issue here, as these two corrections should essentially be the same thing - there are some words that are not on the CDI, and more of these are the low frequency/hard words. So really, it all is a correction for missing words.

Also - the first correction, which depends on the parametric form of the logistic, is much much smaller than the second. Take a look at this.

Equation 3:

\[ p(w_i) = 1 - \frac{1}{1 + e^{\frac{-(i-a)}{b}}} \]

a <- 600 # from figure 6c for a 30mo
b <- 180 # from figure 6c
ranks <- 0:3000
age <- 20
ys <- (1 - (1 / (1 + exp((-(ranks - a ))/b))))

qplot(ranks, ys, 
      geom = "line") + 
  geom_vline(xintercept = 680, lty = 2) +
  geom_polygon(aes(x = c(ranks[ranks > 680], 
                         rev(ranks[ranks > 680])),
                   y = c(ys[ranks > 680], 
                         rep(0, length(ranks[ranks > 680])))), 
               fill = "blue", alpha = .5) + 
  ylim(c(0,1))

So adding the gray area gives us

\[ V_{corr_1} = b \log (1 + e^(a / b)) \]

but then adding the second correction is just a multiplier on this:

\[ V_{corr_2} = \alpha * V_{corr_1} \]

Note that (strikingly), M&P2011 never give their value of \(\alpha\) in the text. I estimate it below so that I can make an estimate of what correction 2 actually looks like…

a <- 600 
b <- 180 
ranks <- 1:3000
age <- 20
ys <- (1 - (1 / (1 + exp((-(ranks - a ))/b))))

area.uncorrected <- sum(ys[1:680])
area.c1 <- b * log(1 + exp(a / b))
area.c2 <- 1142 # from lookup table A1
alpha = area.c2 / area.c1

ys2 <- (1 - (1 / (1 + exp((-(ranks - (a*alpha) ))/(alpha * b)))))
area.corrected <- sum(ys2)

qplot(ranks, ys, 
      geom = "line") + 
  geom_vline(xintercept = 680, lty = 2) +
  geom_polygon(aes(x = c(ranks[ranks > 680], 
                         rev(ranks[ranks > 680])),
                   y = c(ys[ranks > 680], 
                         rep(0, length(ranks[ranks > 680])))), 
               fill = "blue", alpha = .5) + 
  geom_line(aes(y = ys2), lty = 3) + 
  geom_polygon(aes(x = c(ranks, rev(ranks)), 
                y = c(ys2, rev(ys))), 
            fill = "red", 
            alpha = .5) + 
  ylim(c(0,1)) 

So you can see that the second correction dwarfs the first correction in size, and is really based on a few small diary studies.

In sum, I’m worried about this model for a few reasons:

McMurray Model

This is the normal distribution model from the McMurray (2007) “Defusing the Vocabulary Explosion” paper. Assume kids are \(k\) and words \(w\). Kids’ abilities are a function of age (\(a\)). We model production probability \(p\).

\[ k_i \sim \mathcal{N}(\mu_1, \sigma_1) \\ w_j \sim \mathcal{N}(\mu_2, \sigma_2) \\ k_i(a) = k_i * a \\ p(k_i, w_j, a) = k_i(a) > w_j \\ \]

Try this with arbitrary parameters and make the same plot as above.

n.words <- 500
n.kids <- 1000

difficulty <- rnorm(n = n.words, m = 100, sd = 20)

ability <- rnorm(n = n.kids, m = 4, sd = 1) 
age <- 16:30

sims <- expand.grid(difficulty = difficulty, ability = ability, age = age) %>%
  group_by(ability, age) %>%
  mutate(item = 1:n()) %>%
  group_by() %>%
  mutate(produces = difficulty < ability * age) %>%
  group_by(item, age) %>%
  summarise(produces = mean(produces)) %>%
  group_by() %>%
  arrange(age, desc(produces)) %>%
  group_by(age) %>%
  mutate(index = 1:n())
              
qplot(index, produces, facets = ~age, 
      geom = "line", 
      data = sims)

Cool - so this looks almost identical in distributional form! Just for kicks, let’s plot these on top of one another. For now I’m just tweaking parameters, though all four (\(\mu_1\), \(\mu_2\), \(\sigma_1\), and \(\sigma_2\)) matter to the fit.

n.words <- 680 # same as on the CDI
n.kids <- 500 # reasonable number, so the curves are smoothish

difficulty <- rnorm(n = n.words, m = 100, sd = 20)

ability <- rnorm(n = n.kids, m = 4, sd = 1) 
age <- 16:30

sims <- expand.grid(difficulty = difficulty, ability = ability, age = age) %>%
  group_by(ability, age) %>%
  mutate(item = 1:n()) %>%
  group_by() %>%
  mutate(produces = difficulty < ability * age) %>%
  group_by(item, age) %>%
  summarise(produces = mean(produces)) %>%
  group_by() %>%
  arrange(age, desc(produces)) %>%
  group_by(age) %>%
  mutate(index = 1:n())

sims$dataset <- "simulations"
ordered$dataset <- "empirical WS"
d <- bind_rows(select(sims, index, produces, age, dataset), 
               select(ordered, index, produces, age, dataset))

qplot(index, produces, facets = ~age, 
      geom = "line", 
      col = dataset,
      data = d)

Extending the McMurray Model

So if the McMurray model is more or less the right model of vocabulary growth, how do we use knowledge about its parameters to correct CDI data? Two points here.

First, we have to define a process by which words are excluded and then estimate the parameters of the full model based on the CDI model.

Second, to do this we’re still going to need some facts about total vocabulary to compare to the CDI vocabulary estimates, so we’ll still need a study like Robinson & Mervis (1999).

So let’s consider the McMurray model as the base of our generative model, having the steps:

  1. Generate a vocabulary with normally distributed difficulties,
  2. Generate a set of kids with normally distributed learning rates, and
  3. Generate a set of words to be on an instrument.

Let’s consider a model of selecting words for an instrument where the probability of selecting a word gradually decreases with its difficulty rank. The easiest words (e.g. momma, no, ball) will all be on the list, but only a smaller number of hard words will make it.

\[ p_{inclusion}(i) \sim \operatorname{Bern} (\frac{1}{i^{\alpha}}) \]

So each word is included as a function of a Bernoulli draw (coin flip) with probability proportional to its rank.

xs <- 1:100
ys <- 1 / xs^(1/4)
qplot(xs, ys)

So let’s see what this looks like. We’ll do the same generative process as before but this time with many more words, which we’ll exclude at random.

n.words <- 4000
n.kids <- 100
alpha = .25

difficulty <- rnorm(n = n.words, m = 100, sd = 20)
ability <- rnorm(n = n.kids, m = 4, sd = 1) 
age <- 16:30

sims <- expand.grid(difficulty = difficulty, ability = ability, age = age) %>%
  group_by(ability, age) %>%
  mutate(item = 1:n()) %>%
  group_by() %>%
  mutate(produces = difficulty < ability * age) %>%
  group_by(item, age) %>%
  summarise(produces = mean(produces)) %>%
  group_by() %>%
  arrange(age, desc(produces)) %>%
  group_by(age) %>%
  mutate(index = 1:n())

Now choose the words to exclude, based on actual ground-truth difficulty.

words.to.include <- expand.grid(difficulty = difficulty) %>%
  mutate(item = 1:n()) %>%
  arrange(difficulty) %>%
  mutate(diff.idx = 1:n()) %>%
  rowwise %>%
  mutate(inc = rbinom(1, size = 1, prob = 1/diff.idx^alpha) == 1) %>%
  filter(inc) %>%
  rowwise %>%
  mutate(sim.idx = sims$index[sims$item == item][1])

First visualize this with the words, with red being more included words.

qplot(index, produces, facets = ~age, 
      geom = "line", 
      data = sims) + 
  geom_vline(data = words.to.include, 
             aes(xintercept = sim.idx), 
             col = "red", alpha = .03)

Next show the curves with only those words included.

sims.subset <- sims %>%
  filter(item %in% words.to.include$item) %>%
  mutate(index = 1:n())

sims$dataset <- "full simulation"
sims.subset$dataset <- "item subset"

d <- bind_rows(sims, sims.subset)
  
qplot(index, produces, facets = ~age, 
      geom = "line", col = dataset,
      data = d) 

Now plot the multiplier in this simulation, which we can get based on the area under the curve. (As M&P2011 note, the area under the curve is the expected vocabulary estimate for that measure).

ms <- d %>%
  group_by(age, dataset) %>%
  summarise(produces = sum(produces))

qplot(age, produces, col = dataset, 
      geom = "line", data = ms) + 
  geom_hline(yintercept = 680, lty = 2) + 
  geom_hline(yintercept = 3000, lty = 2)

It’s pretty clear that we’re actually getting something similar to Bernard & Mervis (1999) here, and our correction isn’t looking that dissimilar to M&P2011. Of course, this is all with six parameters I made up: \(\mu_1\), \(\mu_2\), \(\sigma_1\), \(\sigma_2\), \(\alpha\) (the parameter on the words that got dropped), and \(N_{total}\) (the total number of words we considered). But we (sort of) fit the first four to the empriical data. And the combo of \(\alpha\) and \(N_{total}\) wasn’t totally invented either.

But really the next step is to use the empirical data and B&M1999 (plus other data) to fit this model, since with the addition of some hyper-priors it’s a well-defined generative model.

Inferring Model Parameters

Let’s start slow, first defining the McMurray model.

mcmurray <-'
data {                            
  int<lower=1> W; // number of words
  int<lower=1> S; // number of subjects
  int<lower=1> N; // number of datapoints (W*S)
  vector[S] age; // subject ages
  int<lower=0,upper=1> produces[N]; // actual data
}

parameters {
  real<lower=0> mu_w; // word mean
  real<lower=0> mu_s; // subject mean
  real<lower=0> sigma_w; // word SD
  real<lower=0> sigma_s; // subject SD
  real<lower=0> difficulty[W]; // word mean
  real<lower=0> ability[S]; // subject mean
}

model {
  mu_w ~ normal(0, 10); 
  mu_s ~ normal(0, 10);
  sigma_w ~ normal(0, 5);
  sigma_s ~ normal(0, 5);
  difficulty ~ normal(mu_w, sigma_w);
  ability ~ normal(mu_s, sigma_s);

  for (s in 1:S)
    for (w in 1:W)
      produces[W*(s-1) + w] ~ bernoulli_logit((ability[s] * age[s]) - difficulty[w]);
}
'

Fake data simulations.

Let’s start by doing simulations with fake data. These are six kids and five words, with the kids gradually increasing in age. Their abilities should be lower and lower, and words 1, 3, and 5 should be estimated as hard.

dat <- list(age = c(12, 14, 16, 18, 20, 22), 
             produces = rep(c(0, 1, 0, 1, 0), 6), 
             W = 5, 
             S = 6, 
             N = 30)

Run!

samps <- pstan(model_code = mcmurray, model_name = "mcmurray_simple", 
             data = dat, iter = 2000, warmup=200, thin=20, chains = 4, 
             pars = c('mu_w','mu_s','sigma_w', 'sigma_s', 
                      'difficulty', 'ability'), 
             verbose = F, refresh=2000/4) 

Now examine output.

samps
## Inference for Stan model: mcmurray_simple.
## 4 chains, each with iter=2000; warmup=200; thin=20; 
## post-warmup draws per chain=90, total post-warmup draws=360.
## 
##                mean se_mean   sd  2.5%   25%   50%   75% 97.5% n_eff Rhat
## mu_w          13.29    0.39 5.25  3.28  9.71 13.07 17.01 24.50   183 1.01
## mu_s           0.67    0.04 0.31  0.25  0.43  0.58  0.90  1.34    55 1.04
## sigma_w        8.30    0.23 2.87  3.60  6.25  7.98  9.89 15.01   157 1.02
## sigma_s        0.17    0.01 0.21  0.02  0.06  0.11  0.20  0.73   291 1.00
## difficulty[1] 21.53    0.44 8.12  9.38 15.70 20.68 26.06 41.14   339 1.00
## difficulty[2]  4.27    0.36 3.50  0.10  1.46  3.38  6.55 12.66    95 1.02
## difficulty[3] 21.18    0.47 8.05 10.08 14.90 19.80 25.74 39.36   290 1.00
## difficulty[4]  4.59    0.52 3.67  0.18  1.99  3.64  6.27 13.32    51 1.04
## difficulty[5] 21.29    0.46 7.98  8.50 15.88 20.32 26.19 38.70   296 1.00
## ability[1]     0.73    0.03 0.37  0.24  0.45  0.64  1.00  1.67   151 1.02
## ability[2]     0.68    0.04 0.32  0.23  0.43  0.60  0.93  1.33    56 1.04
## ability[3]     0.67    0.04 0.32  0.23  0.42  0.59  0.86  1.38    61 1.04
## ability[4]     0.66    0.04 0.31  0.20  0.41  0.60  0.89  1.33    71 1.03
## ability[5]     0.65    0.03 0.30  0.23  0.42  0.56  0.85  1.33    95 1.03
## ability[6]     0.60    0.04 0.28  0.21  0.39  0.53  0.79  1.17    44 1.05
## lp__           2.60    0.72 5.45 -8.57 -0.85  2.41  6.19 12.45    57 1.05
## 
## Samples were drawn using NUTS(diag_e) at Tue Aug  4 21:08:31 2015.
## For each parameter, n_eff is a crude measure of effective sample size,
## and Rhat is the potential scale reduction factor on split chains (at 
## convergence, Rhat=1).

Looks reasonable to me.

Simulations with a subset of the data.

I started out these simulations looking at many words, and a small number of kids, and the results looked funky. Not sure if that’s a convergence issue, or an issue with not having the appropriate diversity of kids. So let’s try a smaller number of words.

n.words <- 6
n.subs <- 500
  
model.data <- ws %>%
  filter(data_id <= n.subs, 
         item %in% c("mommy","dog","up","table","truck","alligator")) %>%
  select(data_id, num_item_id, item, value, age) %>%
  rename(produces = value) %>%
  arrange(data_id) # critical to get the words nested inside subjects

# for simulations, fill in false for missing data
model.data$produces[is.na(model.data$produces)] <- FALSE 

ages <- model.data %>% 
              group_by(data_id) %>% 
              summarise(age = mean(age)) %>% 
              select(age) 

dat <- list(age = ages$age, 
            produces = model.data$produces, 
            W = n.words, 
            S = n.subs, 
            N = n.words * n.subs)

model.data %>%
  group_by(num_item_id) %>%
  summarise(produces = mean(produces))
## Source: local data frame [6 x 2]
## 
##   num_item_id produces
## 1          13    0.264
## 2          26    0.878
## 3          69    0.670
## 4         290    0.448
## 5         366    0.948
## 6         635    0.564

Run the model.

samps <- pstan(model_code = mcmurray, model_name = "mcmurray_simple", 
             data = dat, iter = 2000, warmup=200, thin=20, chains = 4, 
             pars = c('mu_w','mu_s','sigma_w', 'sigma_s', 
                      'difficulty', 'ability'), 
             verbose = F, refresh=2000/4) 

Diagnostics. Order of words is “alligator”, “dog”, “truck”, “table”, “mommy”, “up.”

traceplot(samps, pars = c("mu_w", "mu_s", "sigma_w", "sigma_s"))

samplerpar = get_sampler_params(samps)[[1]]
summary(samplerpar)
##  accept_stat__         stepsize__        treedepth__    n_leapfrog__   
##  Min.   :0.0006583   Min.   :0.007812   Min.   :2.00   Min.   :  3.00  
##  1st Qu.:0.9101136   1st Qu.:0.119155   1st Qu.:5.00   1st Qu.: 31.00  
##  Median :0.9566540   Median :0.119155   Median :5.00   Median : 31.00  
##  Mean   :0.8885530   Mean   :0.123745   Mean   :4.93   Mean   : 31.04  
##  3rd Qu.:0.9815654   3rd Qu.:0.119155   3rd Qu.:5.00   3rd Qu.: 31.00  
##  Max.   :1.0000000   Max.   :0.514698   Max.   :7.00   Max.   :127.00  
##  n_divergent__ 
##  Min.   :0.00  
##  1st Qu.:0.00  
##  Median :0.00  
##  Mean   :0.01  
##  3rd Qu.:0.00  
##  Max.   :1.00
pairs(samps, pars=c('mu_w', 'mu_s'))

Simulations with full dataset

We need to use only complete cases. But even here, these simulations take a while.

n.words <- 680

complete <- ws %>%
  group_by(data_id) %>%
  summarize(complete = sum(!is.na(value)) == n.words) %>%
  filter(complete)

n.subs  <- 10

model.data <- ws %>%
  select(data_id, num_item_id, item, value, age) %>%
  filter(data_id %in% complete$data_id) %>%
  mutate(data_id = as.numeric(as.factor(data_id))) %>% # hack to renumber
  filter(data_id <= n.subs) %>%
  rename(produces = value) %>%
  arrange(data_id) # critical to get the words nested inside subjects

ages <- model.data %>% 
              group_by(data_id) %>% 
              summarise(age = mean(age)) %>% 
              select(age) 

dat <- list(age = ages$age, 
            produces = model.data$produces, 
            W = n.words, 
            S = n.subs, 
            N = n.words * n.subs)

Run the model. Unfortunately this takes around 45 minutes, even for 100 subjects. So do it for 10 (~90s).

samps <- pstan(model_code = mcmurray, model_name = "mcmurray_simple", 
            data = dat, iter = 2000, warmup=200, thin=10, chains = 4, 
             pars = c('mu_w','mu_s','sigma_w', 'sigma_s', 
                      'difficulty', 'ability'), 
             verbose = F, refresh=2000/4) 

Re-merge coefficients with word data.

coefs <- data.frame(summary(samps)$summary)
coefs$name <- rownames(coefs)

word_ids <- ws %>% 
  group_by(num_item_id) %>% 
  summarise(item = unique(item))

words <- coefs %>% 
  filter(str_detect(name, "difficulty")) %>%
  separate(name, c("name", "num_item_id"), "\\[") %>%
  mutate(num_item_id = as.numeric(str_replace(num_item_id, "]", ""))) %>%
  left_join(word_ids) %>%
  select(mean, se_mean, sd, item) %>%
  arrange(mean) %>%
  mutate(item = factor(item, 
                        levels = item,
                        labels = item))

And plot the first 100 words or so… even within this, you can see that there is some good signal here and we are learning good stuff.

qplot(item, mean, data=words[1:100,]) + 
  theme(axis.text.x = element_text(angle = 90, hjust = 1))

Attempt #2: no by-subject coefficients

The problem here is computational complexity. Let’s rewrite the model so that we’re not actually learning anything about each subject, just learning a mean and variance that gets multiplied by age.

mcmurray_nosubs <-'
data {                            
  int<lower=1> W; // number of words
  int<lower=1> S; // number of subjects
  int<lower=1> N; // number of datapoints (W*S)
  vector[S] age; // subject ages
  int<lower=0,upper=1> produces[N]; // actual data
}

parameters {
  real<lower=0> mu_w; // word mean
  real<lower=0> mu_s; // subject mean
  real<lower=0> sigma_w; // word SD
  real<lower=0> difficulty[W]; // word mean
}

model {
  mu_w ~ normal(0, 10); 
  mu_s ~ normal(0, 10);
  sigma_w ~ normal(0, 5);
  difficulty ~ normal(mu_w, sigma_w);

  for (s in 1:S)
    for (w in 1:W)
      produces[W*(s-1) + w] ~ bernoulli_logit((mu_s * age[s]) - difficulty[w]);
}
'

Again, simulations with fake data. to begin These are six kids and five words, with the kids gradually increasing in age. Their abilities should be lower and lower, and words 1, 3, and 5 should be estimated as hard.

dat <- list(age = c(12, 14, 16, 18, 20, 22), 
             produces = rep(c(0, 1, 0, 1, 0), 6), 
             W = 5, 
             S = 6, 
             N = 30)

Run!

samps <- pstan(model_code = mcmurray_nosubs, model_name = "mcmurray_nosubs", 
             data = dat, iter = 2000, warmup=200, thin=20, chains = 4, 
             pars = c('mu_w','mu_s','sigma_w', 'difficulty'), 
             verbose = F, refresh=2000/4) 

Now examine output.

samps
## Inference for Stan model: mcmurray_nosubs.
## 4 chains, each with iter=2000; warmup=200; thin=20; 
## post-warmup draws per chain=90, total post-warmup draws=360.
## 
##                mean se_mean   sd  2.5%   25%   50%   75% 97.5% n_eff Rhat
## mu_w          11.26    0.30 4.91  2.58  7.84 10.87 14.45 22.69   272 1.02
## mu_s           0.57    0.01 0.25  0.21  0.39  0.53  0.71  1.21   360 1.01
## sigma_w        8.00    0.14 2.74  3.75  6.12  7.60  9.42 14.65   360 1.00
## difficulty[1] 19.19    0.41 7.86  7.41 13.80 17.80 22.79 40.05   360 1.00
## difficulty[2]  3.93    0.16 2.93  0.22  1.75  3.32  5.56 10.77   343 1.01
## difficulty[3] 18.86    0.44 8.37  7.53 13.00 17.67 22.12 40.04   360 1.01
## difficulty[4]  3.97    0.16 3.03  0.06  1.63  3.49  5.57 11.19   339 1.01
## difficulty[5] 19.18    0.42 7.94  8.44 13.26 17.63 23.79 37.46   360 1.00
## lp__          -2.83    0.14 2.74 -8.75 -4.38 -2.59 -0.91  1.67   360 1.00
## 
## Samples were drawn using NUTS(diag_e) at Wed Aug  5 09:44:53 2015.
## For each parameter, n_eff is a crude measure of effective sample size,
## and Rhat is the potential scale reduction factor on split chains (at 
## convergence, Rhat=1).

Looks fine. On to a subset of the dataset.

n.words <- 680

complete <- ws %>%
  group_by(data_id) %>%
  summarize(complete = sum(!is.na(value)) == n.words) %>%
  filter(complete)

n.total.subs <- length(unique(ws$data_id))
n.subs  <- 100

model.data <- ws %>%
  select(data_id, num_item_id, item, value, age) %>%
  filter(data_id %in% complete$data_id) %>%
  mutate(data_id = as.numeric(as.factor(data_id))) %>% # hack to renumber
  filter(data_id %in% sample(n.total.subs)[1:n.subs]) %>%
  rename(produces = value) %>%
  arrange(data_id) # critical to get the words nested inside subjects

ages <- model.data %>% 
              group_by(data_id) %>% 
              summarise(age = mean(age)) %>% 
              select(age) 

dat <- list(age = ages$age,
            produces = model.data$produces, 
            W = n.words, 
            S = n.subs, 
            N = n.words * n.subs)

samps <- pstan(model_code = mcmurray_nosubs, model_name = "mcmurray_nosubs", 
            data = dat, iter = 1000, warmup=100, thin=10, chains = 4, 
             pars = c('mu_w','mu_s','sigma_w','difficulty'), 
             verbose = F, refresh=2000/4) 

samps
## Inference for Stan model: mcmurray_nosubs.
## 4 chains, each with iter=1000; warmup=100; thin=10; 
## post-warmup draws per chain=90, total post-warmup draws=360.
## 
##                      mean se_mean     sd      2.5%       25%       50%
## mu_w                 7.41    0.23   0.51      6.30      7.11      7.36
## mu_s                 0.30    0.01   0.02      0.25      0.29      0.30
## sigma_w              1.33    0.03   0.09      1.16      1.25      1.33
## difficulty[1]        5.60    0.20   0.49      4.62      5.30      5.56
## difficulty[2]        5.66    0.21   0.51      4.58      5.33      5.68
## difficulty[3]        7.41    0.22   0.55      6.31      7.04      7.38
## difficulty[4]        4.94    0.22   0.68      3.91      4.69      4.96
## difficulty[5]        4.77    0.25   1.11      0.23      4.56      4.94
## difficulty[6]        3.72    0.64   1.99      0.01      4.04      4.60
## difficulty[7]        5.22    0.19   0.49      4.27      4.88      5.22
## difficulty[8]        5.35    0.20   0.50      4.32      5.01      5.35
## difficulty[9]        4.11    0.22   0.52      3.18      3.70      4.10
## difficulty[10]       5.68    0.18   0.48      4.59      5.40      5.71
## difficulty[11]       5.08    0.20   0.49      4.11      4.78      5.09
## difficulty[12]       4.22    1.72   2.47      0.01      2.94      5.38
## difficulty[13]       7.21    0.20   0.52      6.15      6.87      7.21
## difficulty[14]       6.88    0.38   1.37      0.06      6.72      7.06
## difficulty[15]       7.47    0.21   0.54      6.38      7.10      7.52
## difficulty[16]       5.92    0.19   0.48      4.91      5.61      5.98
## difficulty[17]       6.59    0.23   0.53      5.49      6.25      6.53
## difficulty[18]       5.30    0.20   0.48      4.31      4.97      5.26
## difficulty[19]       5.12    0.72   2.05      0.01      5.42      5.83
## difficulty[20]       6.44    0.20   0.52      5.36      6.11      6.46
## difficulty[21]       6.71    0.23   0.52      5.72      6.34      6.69
## difficulty[22]       5.39    0.21   0.49      4.46      5.08      5.36
## difficulty[23]       6.37    0.21   0.50      5.32      6.03      6.33
## difficulty[24]       5.79    0.20   0.51      4.74      5.48      5.77
## difficulty[25]       8.10    0.23   0.55      7.03      7.76      8.11
## difficulty[26]       4.23    0.18   0.53      3.21      3.86      4.24
## difficulty[27]       8.17    0.24   0.55      7.04      7.80      8.11
## difficulty[28]       4.92    0.17   0.48      3.96      4.58      4.96
## difficulty[29]       6.84    0.20   0.53      5.73      6.50      6.87
## difficulty[30]       5.14    0.67   1.86      0.02      5.31      5.71
## difficulty[31]       6.59    0.22   0.51      5.58      6.23      6.56
## difficulty[32]       7.28    0.22   0.54      6.09      6.95      7.31
## difficulty[33]       8.36    0.24   0.57      7.22      7.98      8.36
## difficulty[34]       9.06    0.23   0.59      7.82      8.69      9.03
## difficulty[35]       6.23    0.33   1.07      5.11      6.01      6.36
## difficulty[36]       5.47    0.19   0.49      4.30      5.18      5.46
## difficulty[37]       8.26    0.23   0.58      6.96      7.86      8.32
## difficulty[38]       6.79    0.20   0.54      5.58      6.47      6.81
## difficulty[39]       6.56    0.20   0.49      5.50      6.28      6.56
## difficulty[40]       8.85    0.22   0.57      7.70      8.46      8.87
## difficulty[41]       6.67    0.19   0.49      5.63      6.34      6.67
## difficulty[42]       7.19    0.22   0.54      6.08      6.85      7.16
## difficulty[43]       8.04    0.24   0.57      6.79      7.66      7.97
## difficulty[44]       6.10    0.17   0.49      5.09      5.76      6.15
## difficulty[45]       8.57    0.24   0.57      7.39      8.16      8.54
## difficulty[46]       6.24    0.20   0.50      5.22      5.91      6.23
## difficulty[47]       8.40    0.24   0.58      7.21      7.99      8.35
## difficulty[48]       6.82    0.20   0.51      5.75      6.47      6.81
## difficulty[49]       7.52    0.22   0.55      6.46      7.17      7.55
## difficulty[50]       7.12    0.19   0.51      6.01      6.83      7.12
## difficulty[51]       7.19    0.21   0.52      6.06      6.84      7.24
## difficulty[52]       7.57    0.20   0.52      6.51      7.20      7.54
## difficulty[53]       6.90    0.21   0.53      5.71      6.58      6.92
## difficulty[54]       8.72    0.24   0.59      7.45      8.32      8.76
## difficulty[55]       7.35    0.23   0.54      6.25      6.98      7.36
## difficulty[56]       6.01    0.19   0.49      5.03      5.70      6.03
## difficulty[57]       6.61    0.21   0.51      5.50      6.27      6.60
## difficulty[58]       6.00    0.21   0.68      4.94      5.69      6.08
## difficulty[59]       6.25    0.19   0.50      5.27      5.94      6.24
## difficulty[60]       4.75    0.25   1.04      0.29      4.61      4.95
## difficulty[61]       7.27    0.20   0.52      6.07      6.95      7.29
## difficulty[62]       7.84    0.22   0.52      6.77      7.51      7.86
## difficulty[63]       7.86    0.23   0.56      6.71      7.49      7.81
## difficulty[64]       9.04    0.24   0.61      7.72      8.66      9.06
## difficulty[65]       7.65    0.22   0.53      6.65      7.32      7.58
## difficulty[66]       7.80    0.23   0.55      6.70      7.48      7.75
## difficulty[67]       6.61    0.20   0.52      5.46      6.26      6.63
## difficulty[68]       8.61    0.22   0.59      7.34      8.22      8.62
## difficulty[69]       5.54    0.18   0.48      4.54      5.23      5.56
## difficulty[70]       3.46    0.32   1.07      0.14      3.27      3.67
## difficulty[71]       5.59    0.19   0.49      4.56      5.30      5.61
## difficulty[72]       7.63    0.20   0.53      6.45      7.30      7.64
## difficulty[73]       6.58    0.20   0.49      5.51      6.27      6.55
## difficulty[74]       5.05    0.19   0.59      4.08      4.79      5.06
## difficulty[75]       5.80    0.19   0.49      4.84      5.49      5.80
## difficulty[76]       8.41    0.22   0.59      7.21      7.96      8.42
## difficulty[77]       7.05    0.20   0.52      5.90      6.70      7.07
## difficulty[78]       6.84    0.20   0.53      5.75      6.50      6.82
## difficulty[79]       7.71    0.22   0.56      6.45      7.36      7.77
## difficulty[80]       8.51    0.24   0.57      7.29      8.13      8.51
## difficulty[81]       7.10    0.21   0.53      5.99      6.75      7.12
## difficulty[82]       7.56    0.20   0.54      6.33      7.24      7.60
## difficulty[83]       8.18    0.21   0.54      7.07      7.86      8.20
## difficulty[84]       7.59    0.21   0.54      6.46      7.26      7.62
## difficulty[85]       7.13    0.21   0.53      5.94      6.81      7.15
## difficulty[86]       7.33    0.21   0.53      6.25      6.97      7.28
## difficulty[87]       6.40    0.19   0.50      5.40      6.09      6.38
## difficulty[88]       5.45    0.16   0.93      3.81      5.23      5.50
## difficulty[89]       7.05    0.36   1.19      5.69      6.85      7.18
## difficulty[90]       5.09    0.19   0.50      4.00      4.72      5.12
## difficulty[91]       7.40    0.22   0.55      6.29      7.08      7.40
## difficulty[92]       6.59    0.21   0.52      5.46      6.24      6.58
## difficulty[93]       7.40    0.23   0.53      6.36      7.05      7.36
## difficulty[94]       6.78    0.19   0.49      5.69      6.47      6.81
## difficulty[95]       5.82    1.01   2.45      0.01      6.33      6.66
## difficulty[96]       7.09    0.21   0.53      5.99      6.75      7.11
## difficulty[97]       6.84    0.25   0.54      5.79      6.47      6.77
## difficulty[98]       7.13    0.21   0.52      6.03      6.83      7.11
## difficulty[99]       5.09    0.62   1.75      0.01      5.22      5.58
## difficulty[100]      6.58    0.19   0.49      5.54      6.25      6.60
## difficulty[101]      7.47    0.20   0.55      6.25      7.14      7.50
## difficulty[102]      7.66    0.23   0.56      6.50      7.35      7.62
## difficulty[103]      8.06    0.21   0.54      6.69      7.73      8.12
## difficulty[104]      5.56    0.19   0.48      4.54      5.26      5.54
## difficulty[105]      7.06    0.20   0.51      6.01      6.78      7.05
## difficulty[106]      5.77    0.22   0.52      4.69      5.46      5.73
## difficulty[107]      7.60    0.22   0.55      6.44      7.28      7.57
## difficulty[108]      6.14    0.18   0.48      5.12      5.84      6.15
## difficulty[109]      6.58    0.20   0.53      5.41      6.30      6.55
## difficulty[110]      6.68    0.21   0.52      5.66      6.30      6.75
## difficulty[111]      7.04    0.22   0.53      5.93      6.70      7.05
## difficulty[112]      6.76    0.19   0.50      5.72      6.42      6.80
## difficulty[113]      6.72    0.20   0.51      5.65      6.40      6.73
## difficulty[114]      8.06    0.23   0.55      6.92      7.71      8.06
## difficulty[115]      7.96    0.21   0.56      6.72      7.57      7.97
## difficulty[116]      7.45    0.21   0.53      6.30      7.11      7.43
## difficulty[117]      6.84    0.21   0.53      5.68      6.47      6.88
## difficulty[118]      6.53    0.21   0.51      5.49      6.21      6.51
## difficulty[119]      8.05    0.22   0.55      6.91      7.70      8.04
## difficulty[120]      8.31    0.21   0.54      7.12      8.00      8.29
## difficulty[121]      4.36    1.71   2.46      0.04      3.27      5.49
## difficulty[122]      8.36    0.22   0.55      7.20      7.98      8.35
## difficulty[123]      7.51    0.23   0.54      6.40      7.16      7.47
## difficulty[124]      8.44    0.23   0.59      7.24      8.05      8.46
## difficulty[125]      3.90    1.60   2.29      0.00      2.55      4.97
## difficulty[126]      7.66    0.23   0.55      6.50      7.28      7.60
## difficulty[127]      7.49    0.23   0.55      6.33      7.11      7.44
## difficulty[128]      8.23    0.22   0.56      7.11      7.85      8.24
## difficulty[129]      6.63    0.19   0.51      5.53      6.25      6.66
## difficulty[130]      7.23    0.23   0.52      6.17      6.94      7.16
## difficulty[131]      7.54    0.22   0.53      6.44      7.25      7.53
## difficulty[132]      7.42    0.21   0.54      6.29      7.07      7.45
## difficulty[133]      7.72    0.26   0.57      6.62      7.33      7.67
## difficulty[134]      6.23    0.21   0.50      5.21      5.92      6.19
## difficulty[135]      7.08    0.20   0.51      5.99      6.75      7.06
## difficulty[136]      7.55    0.21   0.54      6.46      7.18      7.57
## difficulty[137]      7.27    0.20   0.53      6.06      6.91      7.37
## difficulty[138]      7.06    0.22   0.54      5.93      6.75      7.08
## difficulty[139]      7.56    0.21   0.54      6.43      7.22      7.58
## difficulty[140]      8.61    0.24   0.59      7.33      8.20      8.64
## difficulty[141]      7.32    0.24   0.56      6.22      6.95      7.28
## difficulty[142]      7.67    0.21   0.54      6.57      7.31      7.66
## difficulty[143]      8.66    0.23   0.57      7.50      8.28      8.68
## difficulty[144]      7.34    0.21   0.51      6.38      7.01      7.33
## difficulty[145]      8.24    0.21   0.55      7.08      7.88      8.28
## difficulty[146]      7.90    0.24   0.58      6.71      7.54      7.86
## difficulty[147]      7.52    0.22   0.55      6.50      7.14      7.51
## difficulty[148]      6.98    0.20   0.51      5.85      6.66      6.96
## difficulty[149]      7.20    0.22   0.53      6.05      6.88      7.18
## difficulty[150]      7.00    0.21   0.52      5.88      6.67      7.00
## difficulty[151]      9.13    0.23   0.60      7.81      8.77      9.14
## difficulty[152]      9.17    0.22   0.61      7.88      8.83      9.21
## difficulty[153]      8.28    0.24   0.57      7.06      7.91      8.28
## difficulty[154]      6.00    0.20   0.50      5.02      5.70      5.99
## difficulty[155]      7.20    0.22   0.54      6.12      6.85      7.17
## difficulty[156]      8.44    0.24   0.58      7.35      8.02      8.43
## difficulty[157]      7.75    0.22   0.55      6.59      7.40      7.71
## difficulty[158]      7.46    0.21   0.53      6.45      7.06      7.54
## difficulty[159]      7.12    0.21   0.51      6.05      6.79      7.18
## difficulty[160]      6.63    0.21   0.77      5.55      6.35      6.65
## difficulty[161]      6.60    0.20   0.49      5.55      6.30      6.58
## difficulty[162]      5.93    0.20   0.51      4.83      5.61      5.95
## difficulty[163]      7.85    0.20   0.52      6.72      7.53      7.87
## difficulty[164]      7.97    0.23   0.53      6.80      7.63      7.93
## difficulty[165]      5.10    0.21   0.50      4.12      4.72      5.09
## difficulty[166]      7.04    0.22   0.52      5.97      6.67      7.01
## difficulty[167]      7.94    0.22   0.54      6.79      7.56      7.99
## difficulty[168]      8.55    0.23   0.56      7.33      8.20      8.54
## difficulty[169]      7.86    0.22   0.55      6.65      7.55      7.88
## difficulty[170]      7.02    0.19   0.51      5.88      6.71      7.05
## difficulty[171]      6.79    0.22   0.52      5.76      6.43      6.73
## difficulty[172]      9.24    0.21   0.56      8.10      8.87      9.22
## difficulty[173]      6.61    0.20   0.48      5.58      6.29      6.58
## difficulty[174]      3.65    1.34   2.04      0.01      3.56      4.58
## difficulty[175]      7.50    0.22   0.55      6.43      7.13      7.48
## difficulty[176]      8.15    0.24   0.58      6.93      7.76      8.11
## difficulty[177]      8.63    0.22   0.58      7.45      8.24      8.64
## difficulty[178]      9.38    0.23   0.59      8.13      9.01      9.37
## difficulty[179]      6.20    0.19   0.51      5.04      5.90      6.19
## difficulty[180]      7.36    0.21   0.56      6.28      6.97      7.38
## difficulty[181]      9.24    0.22   0.58      8.01      8.87      9.21
## difficulty[182]      8.22    0.23   0.55      7.13      7.86      8.20
## difficulty[183]      7.45    0.23   0.54      6.37      7.10      7.43
## difficulty[184]      8.97    0.23   0.60      7.68      8.59      8.94
## difficulty[185]      6.56    0.19   0.50      5.52      6.27      6.56
## difficulty[186]      6.47    0.20   0.51      5.45      6.17      6.44
## difficulty[187]      6.52    0.21   0.52      5.50      6.17      6.48
## difficulty[188]      7.24    0.21   0.52      6.06      6.91      7.25
## difficulty[189]      7.06    0.21   0.52      5.99      6.71      7.08
## difficulty[190]      5.40    0.17   0.48      4.29      5.09      5.50
## difficulty[191]      4.48    0.66   1.73      0.01      4.63      5.12
## difficulty[192]      7.41    0.20   0.52      6.34      7.06      7.39
## difficulty[193]      6.62    0.20   0.49      5.57      6.31      6.65
## difficulty[194]      6.24    0.21   0.50      5.17      5.93      6.21
## difficulty[195]      5.95    0.19   0.50      4.89      5.62      5.97
## difficulty[196]      6.36    0.22   0.52      5.32      6.03      6.34
## difficulty[197]      6.33    0.25   0.82      5.21      6.09      6.36
## difficulty[198]      6.95    0.22   0.53      5.91      6.62      6.94
## difficulty[199]      6.93    0.21   0.52      5.79      6.63      6.90
## difficulty[200]      7.71    0.22   0.54      6.52      7.41      7.73
## difficulty[201]      6.00    0.18   0.50      4.88      5.69      6.05
## difficulty[202]      5.35    0.18   0.50      4.31      5.00      5.37
## difficulty[203]      4.79    1.01   2.13      0.02      5.20      5.60
## difficulty[204]      7.49    0.21   0.52      6.33      7.13      7.51
## difficulty[205]      7.97    0.22   0.55      6.88      7.62      7.97
## difficulty[206]      6.25    0.20   0.51      5.23      5.90      6.27
## difficulty[207]      6.81    0.20   0.51      5.66      6.49      6.81
## difficulty[208]      6.65    0.19   0.51      5.63      6.34      6.68
## difficulty[209]      6.57    0.21   0.52      5.48      6.22      6.55
## difficulty[210]      8.93    0.21   0.56      7.83      8.56      8.90
## difficulty[211]      7.43    0.21   0.55      6.24      7.05      7.47
## difficulty[212]      6.12    0.19   0.51      5.02      5.81      6.14
## difficulty[213]      6.15    0.21   0.62      5.12      5.86      6.18
## difficulty[214]      6.59    0.22   0.53      5.56      6.26      6.57
## difficulty[215]      6.80    0.21   0.53      5.70      6.47      6.84
## difficulty[216]      7.13    0.23   0.53      6.08      6.76      7.14
## difficulty[217]      6.33    0.34   1.31      0.23      6.21      6.53
## difficulty[218]      7.72    0.23   0.55      6.62      7.40      7.68
## difficulty[219]      7.69    0.21   0.52      6.60      7.35      7.68
## difficulty[220]      7.89    0.21   0.55      6.72      7.54      7.92
## difficulty[221]      7.01    0.20   0.52      5.89      6.68      7.06
## difficulty[222]      7.43    0.21   0.53      6.28      7.09      7.43
## difficulty[223]      4.87    1.06   2.11      0.03      5.13      5.65
## difficulty[224]      7.97    0.23   0.57      6.80      7.59      8.00
## difficulty[225]      6.30    0.20   0.49      5.32      5.98      6.28
## difficulty[226]      7.55    0.21   0.55      6.40      7.23      7.59
## difficulty[227]      7.77    0.24   0.57      6.50      7.41      7.79
## difficulty[228]      5.13    2.10   3.00      0.00      3.92      6.68
## difficulty[229]      7.66    0.22   0.52      6.57      7.30      7.65
## difficulty[230]      7.94    0.23   0.55      6.84      7.57      7.96
## difficulty[231]      6.37    0.20   0.52      5.29      6.06      6.41
## difficulty[232]      7.51    0.21   0.53      6.30      7.17      7.50
## difficulty[233]      8.35    0.22   0.54      7.18      8.01      8.33
## difficulty[234]      5.93    0.20   0.50      4.86      5.65      5.90
## difficulty[235]      7.22    0.22   0.54      6.11      6.87      7.21
## difficulty[236]      7.11    0.22   0.53      6.01      6.76      7.10
## difficulty[237]      8.38    0.24   0.57      7.14      7.99      8.40
## difficulty[238]      8.38    0.24   0.58      7.23      7.97      8.38
## difficulty[239]      7.45    0.21   0.52      6.36      7.13      7.41
## difficulty[240]      7.05    0.20   0.52      5.92      6.72      7.08
## difficulty[241]      7.70    0.21   0.53      6.72      7.34      7.66
## difficulty[242]      7.39    0.22   0.54      6.32      7.02      7.36
## difficulty[243]      6.90    0.21   0.52      5.88      6.57      6.89
## difficulty[244]      8.02    0.23   0.54      6.86      7.67      7.99
## difficulty[245]      7.13    0.20   0.53      5.96      6.83      7.11
## difficulty[246]      7.26    0.21   0.53      6.12      6.93      7.24
## difficulty[247]      8.36    0.23   0.57      7.12      8.05      8.36
## difficulty[248]      7.64    0.24   0.56      6.50      7.29      7.62
## difficulty[249]      6.56    0.21   0.52      5.47      6.23      6.57
## difficulty[250]      4.55    1.86   2.66      0.00      3.32      5.89
## difficulty[251]      7.85    0.22   0.54      6.72      7.49      7.85
## difficulty[252]      6.23    0.28   1.11      1.45      6.01      6.36
## difficulty[253]      7.28    0.21   0.53      6.22      6.92      7.24
## difficulty[254]      6.17    0.27   1.06      4.75      5.94      6.24
## difficulty[255]      6.64    0.20   0.51      5.62      6.28      6.65
## difficulty[256]      6.59    0.30   1.19      0.84      6.44      6.74
## difficulty[257]      9.51    0.22   0.59      8.32      9.13      9.49
## difficulty[258]      7.22    0.21   0.53      6.19      6.86      7.19
## difficulty[259]      9.52    0.23   0.59      8.29      9.12      9.53
## difficulty[260]      7.25    0.21   0.51      6.12      6.92      7.27
## difficulty[261]      9.05    0.24   0.58      7.79      8.68      9.02
## difficulty[262]      7.00    0.21   0.53      5.92      6.68      6.99
## difficulty[263]      6.36    0.19   0.50      5.26      6.10      6.35
## difficulty[264]      6.07    0.20   0.49      5.02      5.77      6.05
## difficulty[265]      7.46    0.22   0.54      6.27      7.11      7.47
## difficulty[266]      9.09    0.23   0.59      7.77      8.71      9.10
## difficulty[267]      6.31    0.19   0.49      5.23      6.01      6.35
## difficulty[268]      7.62    0.20   0.52      6.51      7.27      7.63
## difficulty[269]      7.27    0.22   0.51      6.26      6.94      7.25
## difficulty[270]      7.71    0.24   0.56      6.60      7.34      7.68
## difficulty[271]      5.93    0.19   0.51      4.91      5.60      5.99
## difficulty[272]      7.81    0.23   0.55      6.63      7.48      7.79
## difficulty[273]      8.43    0.24   0.59      7.19      8.02      8.44
## difficulty[274]      8.18    0.22   0.56      7.05      7.83      8.19
## difficulty[275]      7.50    0.21   0.53      6.37      7.19      7.49
## difficulty[276]      7.23    0.21   0.52      6.14      6.91      7.22
## difficulty[277]      7.96    0.23   0.56      6.74      7.62      7.94
## difficulty[278]      8.04    0.23   0.55      6.92      7.70      8.04
## difficulty[279]      8.92    0.22   0.57      7.74      8.58      8.94
## difficulty[280]      8.69    0.23   0.58      7.42      8.32      8.71
## difficulty[281]      6.56    0.19   0.51      5.57      6.20      6.58
## difficulty[282]      7.65    0.22   0.56      6.45      7.30      7.66
## difficulty[283]      7.73    0.22   0.56      6.50      7.36      7.72
## difficulty[284]      7.13    0.20   0.53      5.98      6.81      7.16
## difficulty[285]      7.16    0.19   0.53      5.88      6.83      7.21
## difficulty[286]      7.52    0.22   0.56      6.40      7.12      7.55
## difficulty[287]      9.16    0.23   0.57      8.04      8.79      9.12
## difficulty[288]      7.60    0.21   0.56      6.37      7.24      7.65
## difficulty[289]      7.76    0.21   0.52      6.72      7.44      7.79
## difficulty[290]      7.16    0.20   0.52      6.04      6.83      7.16
## difficulty[291]      6.81    0.21   0.53      5.74      6.42      6.76
## difficulty[292]      7.53    0.22   0.54      6.42      7.16      7.52
## difficulty[293]      7.85    0.22   0.54      6.70      7.51      7.84
## difficulty[294]      8.06    0.23   0.56      6.82      7.72      8.04
## difficulty[295]      7.37    0.22   0.52      6.34      7.00      7.34
## difficulty[296]      7.89    0.20   0.55      6.77      7.54      7.93
## difficulty[297]      5.98    0.20   0.52      4.83      5.71      5.97
## difficulty[298]      8.93    0.23   0.59      7.66      8.58      8.94
## difficulty[299]      7.27    0.22   0.53      6.12      6.92      7.31
## difficulty[300]      8.25    0.21   0.55      7.07      7.87      8.28
## difficulty[301]      7.87    0.22   0.53      6.65      7.53      7.86
## difficulty[302]      8.15    0.24   0.55      7.03      7.81      8.13
## difficulty[303]      6.13    0.18   0.51      5.05      5.82      6.11
## difficulty[304]      6.95    0.21   0.52      5.77      6.65      6.96
## difficulty[305]      6.56    0.20   0.51      5.47      6.21      6.59
## difficulty[306]      6.76    0.21   0.52      5.63      6.44      6.75
## difficulty[307]      8.51    0.21   0.56      7.38      8.13      8.55
## difficulty[308]      8.19    0.22   0.54      7.08      7.80      8.25
## difficulty[309]      7.66    0.21   0.53      6.55      7.32      7.68
## difficulty[310]      8.08    0.22   0.53      6.97      7.76      8.05
## difficulty[311]      7.06    0.20   0.52      5.93      6.72      7.06
## difficulty[312]      6.72    0.21   0.52      5.57      6.39      6.71
## difficulty[313]      7.52    0.21   0.54      6.39      7.18      7.50
## difficulty[314]      8.18    0.23   0.58      6.89      7.81      8.25
## difficulty[315]      8.82    0.23   0.58      7.68      8.44      8.75
## difficulty[316]      6.56    0.22   0.52      5.59      6.22      6.52
## difficulty[317]      7.20    0.20   0.53      6.04      6.85      7.28
## difficulty[318]      9.03    0.22   0.57      7.76      8.72      9.03
## difficulty[319]      7.62    0.22   0.54      6.47      7.28      7.62
## difficulty[320]      6.90    0.21   0.51      5.84      6.57      6.93
## difficulty[321]      6.78    0.20   0.51      5.68      6.44      6.85
## difficulty[322]      5.96    0.18   0.50      4.88      5.66      6.00
## difficulty[323]      6.13    0.20   0.50      5.18      5.81      6.09
## difficulty[324]      7.76    0.23   0.55      6.57      7.41      7.73
## difficulty[325]      8.14    0.24   0.56      7.03      7.75      8.12
## difficulty[326]      9.43    0.22   0.58      8.31      9.05      9.42
## difficulty[327]      8.00    0.23   0.56      6.84      7.62      8.00
## difficulty[328]      9.04    0.25   0.60      7.88      8.62      9.05
## difficulty[329]     10.14    0.21   0.60      8.87      9.79     10.16
## difficulty[330]      9.27    0.22   0.58      7.99      8.91      9.30
## difficulty[331]      8.27    0.24   0.56      7.20      7.87      8.23
## difficulty[332]      8.44    0.21   0.55      7.20      8.15      8.45
## difficulty[333]      6.13    0.19   0.50      5.01      5.82      6.12
## difficulty[334]      6.79    0.20   0.52      5.71      6.46      6.74
## difficulty[335]      7.86    0.21   0.53      6.75      7.54      7.87
## difficulty[336]      5.87    0.18   0.50      4.75      5.57      5.91
## difficulty[337]      6.79    0.19   0.52      5.72      6.47      6.81
## difficulty[338]      7.55    0.22   0.53      6.50      7.18      7.58
## difficulty[339]      9.21    0.22   0.59      7.90      8.81      9.26
## difficulty[340]      7.71    0.22   0.54      6.50      7.38      7.68
## difficulty[341]      6.87    0.21   0.53      5.65      6.56      6.88
## difficulty[342]      7.03    0.22   0.52      5.98      6.72      6.97
## difficulty[343]      9.32    0.23   0.59      8.06      8.94      9.35
## difficulty[344]      6.82    0.21   0.51      5.84      6.47      6.77
## difficulty[345]      8.37    0.25   0.59      7.24      7.99      8.30
## difficulty[346]      7.23    0.21   0.52      6.08      6.94      7.19
## difficulty[347]      7.53    0.22   0.54      6.29      7.21      7.57
## difficulty[348]      4.66    0.18   0.60      3.53      4.35      4.65
## difficulty[349]      9.21    0.25   0.60      7.99      8.85      9.16
## difficulty[350]      7.21    0.23   0.54      6.06      6.86      7.18
## difficulty[351]      7.14    0.20   0.52      5.94      6.80      7.16
## difficulty[352]      7.44    0.20   0.55      6.23      7.08      7.46
## difficulty[353]      9.24    0.23   0.57      8.09      8.89      9.24
## difficulty[354]      7.96    0.22   0.55      6.84      7.59      7.96
## difficulty[355]      8.78    0.22   0.57      7.54      8.43      8.80
## difficulty[356]      2.85    0.23   0.98      0.20      2.55      3.01
## difficulty[357]      7.44    0.22   0.53      6.45      7.10      7.41
## difficulty[358]      8.21    0.22   0.55      7.15      7.89      8.20
## difficulty[359]      7.85    0.21   0.54      6.77      7.52      7.86
## difficulty[360]      7.11    0.22   0.54      6.03      6.78      7.10
## difficulty[361]      5.23    0.21   0.52      4.21      4.91      5.17
## difficulty[362]      5.61    0.19   0.50      4.59      5.26      5.61
## difficulty[363]      8.10    0.23   0.56      6.97      7.74      8.07
## difficulty[364]      8.74    0.24   0.58      7.60      8.35      8.69
## difficulty[365]      7.45    0.22   0.54      6.34      7.11      7.45
## difficulty[366]      1.22    0.70   1.49      0.03      0.10      0.26
## difficulty[367]      9.54    0.23   0.61      8.14      9.19      9.58
## difficulty[368]      6.33    0.21   0.51      5.31      5.96      6.34
## difficulty[369]      7.91    0.24   0.54      6.86      7.56      7.87
## difficulty[370]      9.25    0.22   0.59      7.98      8.89      9.27
## difficulty[371]      5.02    2.05   2.93      0.00      3.87      6.44
## difficulty[372]      8.44    0.23   0.57      7.28      8.11      8.45
## difficulty[373]      7.96    0.21   0.54      6.82      7.61      7.98
## difficulty[374]      8.29    0.22   0.54      7.18      7.97      8.29
## difficulty[375]      7.63    0.20   0.51      6.52      7.28      7.65
## difficulty[376]      5.18    0.18   0.48      4.18      4.88      5.22
## difficulty[377]      7.23    0.21   0.51      6.11      6.94      7.25
## difficulty[378]      2.99    1.18   1.88      0.03      0.15      3.85
## difficulty[379]      6.74    0.21   0.52      5.69      6.43      6.68
## difficulty[380]      7.13    0.21   0.53      6.02      6.78      7.14
## difficulty[381]      7.34    0.22   0.55      6.12      7.02      7.38
## difficulty[382]      7.36    0.22   0.52      6.30      7.04      7.35
## difficulty[383]      6.67    0.19   0.48      5.74      6.35      6.71
## difficulty[384]      5.22    0.14   0.80      3.92      4.99      5.25
## difficulty[385]      4.43    0.46   1.40      0.04      4.43      4.79
## difficulty[386]      7.07    0.22   0.54      5.93      6.72      7.07
## difficulty[387]      6.46    0.21   0.52      5.32      6.15      6.48
## difficulty[388]      4.88    0.50   1.45      0.05      4.86      5.20
## difficulty[389]      4.00    0.17   0.64      2.68      3.67      4.12
## difficulty[390]      7.29    0.20   0.53      6.17      6.95      7.27
## difficulty[391]      5.31    0.43   1.27      0.14      5.24      5.55
## difficulty[392]      4.24    1.73   2.48      0.01      3.13      5.41
## difficulty[393]      5.37    0.20   0.48      4.41      5.06      5.31
## difficulty[394]      7.58    0.23   0.56      6.30      7.24      7.58
## difficulty[395]      7.21    0.22   0.53      6.05      6.85      7.22
## difficulty[396]      7.92    0.23   0.56      6.72      7.56      7.92
## difficulty[397]      3.73    1.58   2.68      0.00      0.03      5.39
## difficulty[398]      7.43    0.21   0.52      6.32      7.12      7.45
## difficulty[399]      7.69    0.23   0.53      6.57      7.33      7.68
## difficulty[400]      5.24    0.18   0.53      4.21      4.94      5.28
## difficulty[401]      6.46    0.22   0.51      5.38      6.11      6.41
## difficulty[402]      7.04    0.20   0.51      5.93      6.70      7.08
## difficulty[403]      7.36    0.21   0.53      6.23      6.99      7.42
## difficulty[404]      7.81    0.22   0.53      6.69      7.51      7.80
## difficulty[405]      8.09    0.22   0.56      6.92      7.73      8.10
## difficulty[406]      7.76    0.21   0.54      6.62      7.42      7.75
## difficulty[407]      8.12    0.21   0.54      7.00      7.78      8.12
## difficulty[408]      7.60    0.21   0.52      6.44      7.25      7.65
## difficulty[409]      7.58    0.24   0.56      6.44      7.21      7.56
## difficulty[410]      8.04    0.23   0.53      7.04      7.71      7.99
## difficulty[411]      6.74    0.19   0.52      5.61      6.40      6.81
## difficulty[412]      5.41    2.21   3.16      0.00      4.01      7.03
## difficulty[413]      7.54    0.22   0.53      6.45      7.18      7.52
## difficulty[414]      7.23    0.20   0.53      6.09      6.91      7.25
## difficulty[415]      7.17    0.20   0.52      5.98      6.85      7.16
## difficulty[416]      8.12    0.23   0.57      6.96      7.76      8.14
## difficulty[417]      6.91    0.22   0.54      5.74      6.55      6.90
## difficulty[418]      7.87    0.22   0.55      6.79      7.49      7.89
## difficulty[419]      6.81    0.20   0.52      5.67      6.49      6.86
## difficulty[420]      7.51    0.22   0.54      6.36      7.18      7.49
## difficulty[421]      5.64    0.54   1.59      0.01      5.69      5.95
## difficulty[422]      7.29    0.22   0.55      6.12      6.92      7.29
## difficulty[423]      7.63    0.22   0.56      6.48      7.26      7.63
## difficulty[424]      7.73    0.23   0.56      6.62      7.39      7.65
## difficulty[425]      8.70    0.24   0.58      7.48      8.36      8.69
## difficulty[426]      5.55    0.35   1.26      0.09      5.47      5.78
## difficulty[427]      7.15    0.22   0.54      5.98      6.79      7.16
## difficulty[428]      7.87    0.21   0.55      6.67      7.56      7.87
## difficulty[429]      7.53    0.21   0.56      6.32      7.17      7.55
## difficulty[430]      8.28    0.24   0.59      7.00      7.89      8.30
## difficulty[431]      8.27    0.23   0.58      7.07      7.89      8.27
## difficulty[432]      7.33    0.21   0.54      6.12      7.03      7.34
## difficulty[433]      6.97    0.20   0.51      5.94      6.66      6.97
## difficulty[434]      7.58    0.23   0.55      6.45      7.21      7.55
## difficulty[435]      5.57    0.18   0.50      4.50      5.21      5.61
## difficulty[436]      9.42    0.24   0.60      8.20      9.03      9.40
## difficulty[437]      7.57    0.22   0.55      6.38      7.20      7.54
## difficulty[438]      7.65    0.20   0.54      6.40      7.30      7.67
## difficulty[439]      6.81    0.21   0.52      5.74      6.49      6.81
## difficulty[440]      7.39    0.21   0.54      6.20      7.02      7.44
## difficulty[441]      7.37    0.21   0.53      6.33      7.00      7.38
## difficulty[442]      7.45    0.22   0.54      6.31      7.12      7.42
## difficulty[443]      6.24    0.43   1.35      0.16      6.12      6.43
## difficulty[444]      7.84    0.22   0.53      6.72      7.50      7.81
## difficulty[445]      7.04    0.22   0.55      5.91      6.67      6.96
## difficulty[446]      7.01    0.21   0.54      5.85      6.68      7.03
## difficulty[447]      6.25    0.27   1.03      5.19      6.04      6.35
## difficulty[448]      7.52    0.23   0.54      6.38      7.18      7.55
## difficulty[449]      7.81    0.23   0.55      6.71      7.45      7.75
## difficulty[450]      7.57    0.22   0.56      6.42      7.21      7.59
## difficulty[451]      8.05    0.22   0.54      6.93      7.71      8.07
## difficulty[452]      6.89    0.23   0.88      5.77      6.61      6.96
## difficulty[453]      6.48    0.41   1.48      0.09      6.41      6.79
## difficulty[454]      7.60    0.24   0.56      6.39      7.27      7.59
## difficulty[455]      6.20    0.32   1.18      0.24      6.05      6.36
## difficulty[456]      8.10    0.23   0.56      6.91      7.73      8.05
## difficulty[457]      8.19    0.22   0.56      6.97      7.86      8.19
## difficulty[458]      6.43    0.20   0.50      5.39      6.11      6.41
## difficulty[459]      8.22    0.24   0.56      6.92      7.91      8.19
## difficulty[460]      9.35    0.24   0.60      8.07      8.96      9.32
## difficulty[461]      7.94    0.22   0.54      6.77      7.62      7.94
## difficulty[462]      7.24    0.21   0.53      6.14      6.92      7.19
## difficulty[463]      7.72    0.22   0.55      6.60      7.37      7.71
## difficulty[464]      6.83    0.24   0.51      5.80      6.49      6.78
## difficulty[465]      6.88    0.23   0.55      5.72      6.51      6.89
## difficulty[466]      9.01    0.23   0.58      7.83      8.66      8.99
## difficulty[467]      7.34    0.22   0.54      6.19      7.03      7.29
## difficulty[468]      7.84    0.21   0.54      6.74      7.47      7.88
## difficulty[469]      6.58    0.24   0.53      5.56      6.22      6.58
## difficulty[470]      7.68    0.23   0.54      6.55      7.34      7.64
## difficulty[471]      7.90    0.21   0.54      6.64      7.59      7.92
## difficulty[472]      8.10    0.23   0.57      6.93      7.75      8.03
## difficulty[473]      6.94    0.20   0.51      5.77      6.62      6.97
## difficulty[474]      6.39    0.21   0.52      5.39      6.05      6.35
## difficulty[475]      9.27    0.23   0.58      7.97      8.93      9.30
## difficulty[476]      6.51    0.20   0.50      5.51      6.15      6.49
## difficulty[477]      7.10    0.21   0.53      5.98      6.74      7.11
## difficulty[478]      7.76    0.22   0.53      6.60      7.41      7.71
## difficulty[479]      7.66    0.22   0.54      6.60      7.32      7.63
## difficulty[480]      7.51    0.21   0.54      6.30      7.16      7.58
## difficulty[481]      7.57    0.22   0.55      6.38      7.21      7.55
## difficulty[482]      7.55    0.23   0.54      6.45      7.20      7.49
## difficulty[483]      6.39    0.19   0.49      5.33      6.10      6.36
## difficulty[484]      7.46    0.20   0.54      6.35      7.16      7.47
## difficulty[485]      7.28    0.21   0.52      6.12      6.97      7.25
## difficulty[486]      6.88    0.21   0.50      5.83      6.54      6.88
## difficulty[487]      7.76    0.21   0.56      6.61      7.38      7.84
## difficulty[488]      7.45    0.23   0.53      6.38      7.11      7.42
## difficulty[489]      8.13    0.23   0.54      7.02      7.78      8.08
## difficulty[490]      8.73    0.24   0.58      7.54      8.35      8.72
## difficulty[491]      9.06    0.24   0.59      7.83      8.71      9.05
## difficulty[492]      7.12    0.21   0.50      6.12      6.79      7.09
## difficulty[493]      6.85    0.20   0.51      5.76      6.54      6.88
## difficulty[494]      7.39    0.21   0.54      6.19      7.05      7.40
## difficulty[495]      7.80    0.23   0.57      6.60      7.46      7.76
## difficulty[496]      7.85    0.22   0.54      6.69      7.55      7.83
## difficulty[497]      6.73    0.20   0.53      5.65      6.38      6.78
## difficulty[498]      7.09    0.23   0.53      6.03      6.74      7.12
## difficulty[499]      7.38    0.24   0.56      6.23      6.97      7.34
## difficulty[500]      7.61    0.20   0.54      6.42      7.25      7.65
## difficulty[501]     10.33    0.22   0.63      9.11      9.88     10.36
## difficulty[502]      7.02    0.22   0.53      6.04      6.67      6.99
## difficulty[503]      7.93    0.23   0.55      6.80      7.58      7.95
## difficulty[504]      5.70    0.17   0.49      4.70      5.36      5.70
## difficulty[505]      6.85    0.20   0.52      5.82      6.50      6.86
## difficulty[506]      7.61    0.22   0.56      6.46      7.26      7.58
## difficulty[507]      6.97    0.20   0.53      5.88      6.60      6.96
## difficulty[508]      8.04    0.22   0.54      6.85      7.69      8.04
## difficulty[509]      6.54    0.22   0.51      5.55      6.23      6.48
## difficulty[510]      7.79    0.21   0.53      6.71      7.46      7.76
## difficulty[511]      6.69    0.22   0.50      5.73      6.36      6.62
## difficulty[512]      7.07    0.22   0.55      5.87      6.70      7.06
## difficulty[513]      8.21    0.23   0.56      7.06      7.87      8.18
## difficulty[514]      7.58    0.21   0.54      6.44      7.25      7.58
## difficulty[515]      7.09    0.22   0.52      6.01      6.78      7.02
## difficulty[516]      6.01    0.21   0.51      4.97      5.68      5.99
## difficulty[517]      7.80    0.21   0.53      6.64      7.48      7.80
## difficulty[518]      7.91    0.23   0.56      6.78      7.53      7.91
## difficulty[519]      6.62    0.21   0.51      5.61      6.28      6.64
## difficulty[520]      7.44    0.25   0.56      6.39      7.04      7.39
## difficulty[521]      7.76    0.22   0.56      6.65      7.41      7.74
## difficulty[522]      7.61    0.23   0.55      6.50      7.22      7.61
## difficulty[523]      9.27    0.22   0.60      8.01      8.89      9.31
## difficulty[524]      8.30    0.21   0.55      7.12      7.94      8.34
## difficulty[525]      8.27    0.21   0.55      7.08      7.91      8.31
## difficulty[526]      7.90    0.22   0.55      6.72      7.55      7.86
## difficulty[527]      6.94    0.22   0.52      5.86      6.60      6.91
## difficulty[528]      7.60    0.22   0.53      6.50      7.26      7.61
## difficulty[529]      7.27    0.22   0.53      6.12      6.91      7.24
## difficulty[530]      8.15    0.24   0.56      6.96      7.78      8.13
## difficulty[531]      7.53    0.21   0.54      6.33      7.18      7.57
## difficulty[532]      7.56    0.23   0.53      6.40      7.21      7.53
## difficulty[533]      4.89    0.17   0.48      3.82      4.64      4.89
## difficulty[534]      7.21    0.22   0.54      6.11      6.88      7.22
## difficulty[535]      6.99    0.21   0.52      5.83      6.68      6.97
## difficulty[536]      9.02    0.21   0.57      7.74      8.62      9.09
## difficulty[537]      7.68    0.21   0.54      6.61      7.36      7.67
## difficulty[538]      8.73    0.22   0.55      7.55      8.40      8.72
## difficulty[539]      7.67    0.21   0.52      6.61      7.33      7.64
## difficulty[540]      8.36    0.22   0.55      7.20      7.99      8.38
## difficulty[541]      9.08    0.23   0.60      7.83      8.71      8.99
## difficulty[542]      8.16    0.21   0.55      6.99      7.83      8.15
## difficulty[543]      7.18    0.21   0.52      6.08      6.84      7.20
## difficulty[544]      8.70    0.22   0.58      7.35      8.33      8.71
## difficulty[545]      8.64    0.24   0.56      7.40      8.29      8.62
## difficulty[546]      7.32    0.23   0.54      6.19      6.96      7.28
## difficulty[547]      9.36    0.21   0.58      8.07      9.01      9.40
## difficulty[548]      6.64    0.21   0.51      5.60      6.32      6.64
## difficulty[549]      7.84    0.20   0.54      6.67      7.53      7.85
## difficulty[550]      7.41    0.21   0.53      6.26      7.06      7.42
## difficulty[551]      7.62    0.22   0.55      6.52      7.27      7.64
## difficulty[552]      7.76    0.22   0.54      6.70      7.39      7.76
## difficulty[553]      7.81    0.25   0.57      6.66      7.44      7.75
## difficulty[554]      7.59    0.23   0.58      6.31      7.26      7.62
## difficulty[555]      8.58    0.23   0.59      7.39      8.19      8.60
## difficulty[556]      7.37    0.20   0.52      6.27      7.06      7.40
## difficulty[557]      7.59    0.22   0.55      6.40      7.27      7.61
## difficulty[558]      7.01    0.20   0.53      5.85      6.64      6.98
## difficulty[559]      7.75    0.21   0.52      6.71      7.42      7.77
## difficulty[560]      8.98    0.25   0.60      7.84      8.56      8.95
## difficulty[561]      7.72    0.21   0.53      6.60      7.40      7.73
## difficulty[562]      6.53    0.22   0.53      5.45      6.20      6.53
## difficulty[563]      8.04    0.23   0.55      6.82      7.68      8.00
## difficulty[564]      8.22    0.21   0.55      6.99      7.90      8.22
## difficulty[565]      7.16    0.21   0.50      6.12      6.87      7.10
## difficulty[566]      6.54    0.20   0.51      5.50      6.19      6.54
## difficulty[567]      8.68    0.22   0.56      7.30      8.31      8.69
## difficulty[568]      9.45    0.22   0.59      8.16      9.08      9.45
## difficulty[569]      8.09    0.22   0.56      6.98      7.75      8.09
## difficulty[570]      8.11    0.21   0.53      6.93      7.78      8.12
## difficulty[571]      7.97    0.21   0.56      6.81      7.62      7.97
## difficulty[572]      7.06    0.21   0.53      5.95      6.73      7.07
## difficulty[573]      7.53    0.21   0.52      6.38      7.19      7.55
## difficulty[574]      9.14    0.24   0.59      7.89      8.70      9.15
## difficulty[575]      8.79    0.23   0.58      7.58      8.43      8.79
## difficulty[576]      8.45    0.22   0.57      7.18      8.08      8.46
## difficulty[577]      8.88    0.26   0.61      7.70      8.49      8.86
## difficulty[578]      9.43    0.23   0.63      8.25      9.03      9.39
## difficulty[579]      8.34    0.23   0.56      7.12      8.00      8.32
## difficulty[580]      8.83    0.22   0.57      7.74      8.43      8.79
## difficulty[581]      9.54    0.24   0.61      8.37      9.15      9.53
## difficulty[582]      8.82    0.24   0.59      7.46      8.45      8.82
## difficulty[583]      8.82    0.25   0.59      7.67      8.43      8.79
## difficulty[584]      7.33    0.20   0.53      6.17      7.00      7.38
## difficulty[585]      7.70    0.23   0.55      6.46      7.36      7.70
## difficulty[586]      6.27    0.20   0.50      5.29      5.96      6.26
## difficulty[587]      5.87    0.21   0.50      4.89      5.51      5.83
## difficulty[588]      7.17    0.21   0.52      5.99      6.83      7.18
## difficulty[589]      8.83    0.23   0.60      7.52      8.48      8.84
## difficulty[590]      9.25    0.22   0.58      8.03      8.88      9.27
## difficulty[591]      8.39    0.22   0.55      7.17      8.06      8.38
## difficulty[592]      6.89    0.19   0.50      5.83      6.59      6.90
## difficulty[593]      9.69    0.24   0.61      8.49      9.29      9.73
## difficulty[594]      9.00    0.23   0.57      7.76      8.64      9.01
## difficulty[595]      8.77    0.23   0.57      7.60      8.40      8.76
## difficulty[596]      9.22    0.25   0.61      7.95      8.80      9.24
## difficulty[597]      7.36    0.20   0.53      6.25      7.00      7.42
## difficulty[598]      8.77    0.22   0.57      7.58      8.44      8.76
## difficulty[599]      9.51    0.24   0.61      8.27      9.14      9.47
## difficulty[600]      8.97    0.21   0.59      7.69      8.60      9.00
## difficulty[601]      7.45    0.23   0.54      6.32      7.07      7.46
## difficulty[602]      8.49    0.22   0.56      7.30      8.15      8.50
## difficulty[603]     10.36    0.19   0.64      9.04      9.95     10.43
## difficulty[604]      9.23    0.24   0.60      8.03      8.82      9.20
## difficulty[605]      7.15    0.22   0.54      6.04      6.78      7.18
## difficulty[606]      9.19    0.22   0.59      7.83      8.84      9.25
## difficulty[607]      7.54    0.21   0.56      6.34      7.16      7.60
## difficulty[608]      9.42    0.25   0.61      8.18      9.04      9.40
## difficulty[609]      8.40    0.25   0.57      7.24      8.02      8.39
## difficulty[610]      8.26    0.24   0.57      7.06      7.90      8.27
## difficulty[611]     10.00    0.22   0.63      8.73      9.59     10.04
## difficulty[612]      9.94    0.24   0.62      8.53      9.53     10.03
## difficulty[613]      9.06    0.23   0.58      7.80      8.71      9.04
## difficulty[614]      8.27    0.23   0.59      7.06      7.89      8.30
## difficulty[615]      8.02    0.22   0.56      6.76      7.68      7.99
## difficulty[616]      7.68    0.26   0.56      6.53      7.31      7.64
## difficulty[617]      8.68    0.23   0.59      7.42      8.32      8.70
## difficulty[618]      9.66    0.23   0.61      8.33      9.24      9.73
## difficulty[619]      8.85    0.23   0.58      7.63      8.51      8.83
## difficulty[620]      5.97    0.19   0.49      4.97      5.59      6.01
## difficulty[621]      8.51    0.22   0.57      7.23      8.15      8.52
## difficulty[622]      7.28    0.22   0.54      6.21      6.91      7.31
## difficulty[623]      6.93    0.21   0.51      5.88      6.59      6.94
## difficulty[624]      9.61    0.24   0.59      8.41      9.24      9.58
## difficulty[625]      9.07    0.24   0.60      7.93      8.64      9.06
## difficulty[626]      9.33    0.23   0.58      8.05      8.98      9.32
## difficulty[627]      6.36    0.19   0.50      5.29      6.06      6.35
## difficulty[628]      6.48    0.20   0.52      5.34      6.14      6.54
## difficulty[629]      8.56    0.23   0.56      7.49      8.18      8.60
## difficulty[630]      6.74    0.19   0.49      5.59      6.47      6.75
## difficulty[631]      8.15    0.25   0.58      6.87      7.80      8.12
## difficulty[632]      7.58    0.22   0.56      6.44      7.21      7.61
## difficulty[633]      8.01    0.21   0.54      6.84      7.66      8.02
## difficulty[634]      7.74    0.24   0.55      6.67      7.35      7.76
## difficulty[635]      5.91    0.19   0.50      4.93      5.51      5.96
## difficulty[636]      8.38    0.22   0.57      7.21      8.04      8.37
## difficulty[637]      7.68    0.20   0.55      6.54      7.36      7.75
## difficulty[638]      7.64    0.21   0.55      6.39      7.32      7.65
## difficulty[639]      8.93    0.23   0.61      7.67      8.50      8.97
## difficulty[640]     10.47    0.25   0.65      9.27     10.06     10.47
## difficulty[641]      8.25    0.22   0.56      7.03      7.91      8.25
## difficulty[642]      8.88    0.23   0.57      7.60      8.52      8.89
## difficulty[643]     10.00    0.24   0.60      8.82      9.59      9.95
## difficulty[644]      9.33    0.22   0.62      7.95      8.99      9.36
## difficulty[645]      5.62    0.25   0.89      4.59      5.42      5.67
## difficulty[646]      8.91    0.23   0.59      7.59      8.55      8.92
## difficulty[647]      9.10    0.24   0.59      7.80      8.72      9.09
## difficulty[648]      8.44    0.24   0.57      7.34      8.09      8.45
## difficulty[649]      8.53    0.21   0.56      7.37      8.15      8.56
## difficulty[650]      8.80    0.25   0.59      7.64      8.38      8.74
## difficulty[651]      7.76    0.20   0.52      6.66      7.42      7.76
## difficulty[652]      8.16    0.21   0.55      6.94      7.84      8.17
## difficulty[653]      7.59    0.22   0.56      6.31      7.25      7.61
## difficulty[654]      8.69    0.23   0.57      7.50      8.32      8.70
## difficulty[655]      8.73    0.22   0.57      7.51      8.40      8.68
## difficulty[656]      9.09    0.22   0.58      7.95      8.71      9.09
## difficulty[657]      8.46    0.23   0.58      7.20      8.12      8.46
## difficulty[658]      9.91    0.23   0.65      8.57      9.47      9.98
## difficulty[659]      8.51    0.21   0.57      7.33      8.15      8.51
## difficulty[660]      7.31    0.20   0.51      6.16      6.99      7.33
## difficulty[661]      9.70    0.22   0.59      8.53      9.34      9.75
## difficulty[662]      7.66    0.23   0.54      6.50      7.32      7.64
## difficulty[663]      8.10    0.21   0.55      6.95      7.75      8.15
## difficulty[664]      8.86    0.23   0.58      7.59      8.46      8.87
## difficulty[665]      8.69    0.23   0.57      7.48      8.33      8.70
## difficulty[666]      8.05    0.23   0.57      6.76      7.66      8.01
## difficulty[667]      7.90    0.21   0.54      6.62      7.58      7.89
## difficulty[668]      8.23    0.21   0.56      7.09      7.88      8.26
## difficulty[669]      8.31    0.23   0.56      7.08      7.92      8.31
## difficulty[670]      7.18    0.20   0.53      6.13      6.85      7.21
## difficulty[671]      9.35    0.23   0.60      8.11      8.98      9.34
## difficulty[672]      9.80    0.21   0.60      8.54      9.40      9.88
## difficulty[673]      9.15    0.25   0.60      7.83      8.77      9.15
## difficulty[674]     10.36    0.20   0.67      8.87      9.90     10.43
## difficulty[675]      8.42    0.23   0.54      7.35      8.10      8.40
## difficulty[676]      8.51    0.22   0.55      7.38      8.15      8.53
## difficulty[677]      9.35    0.23   0.58      8.11      9.00      9.38
## difficulty[678]      9.93    0.21   0.62      8.51      9.49     10.02
## difficulty[679]      9.14    0.23   0.57      7.97      8.78      9.15
## difficulty[680]      9.58    0.24   0.59      8.29      9.21      9.56
## lp__            -30505.97  324.95 726.44 -32156.05 -31010.75 -30254.29
##                       75%     97.5% n_eff  Rhat
## mu_w                 7.92      8.15     5  2.18
## mu_s                 0.32      0.33     5  2.20
## sigma_w              1.39      1.48     7  1.80
## difficulty[1]        5.98      6.49     6  1.62
## difficulty[2]        6.02      6.57     6  1.68
## difficulty[3]        7.88      8.36     6  1.77
## difficulty[4]        5.39      5.87    10  1.28
## difficulty[5]        5.35      6.00    20  1.16
## difficulty[6]        4.98      5.59    10  1.74
## difficulty[7]        5.57      6.09     7  1.45
## difficulty[8]        5.71      6.22     6  1.54
## difficulty[9]        4.48      5.09     6  1.52
## difficulty[10]       6.02      6.50     7  1.54
## difficulty[11]       5.48      5.95     6  1.53
## difficulty[12]       5.93      6.49     2  9.73
## difficulty[13]       7.62      8.15     7  1.67
## difficulty[14]       7.51      8.03    13  1.22
## difficulty[15]       7.86      8.47     6  1.75
## difficulty[16]       6.24      6.76     7  1.59
## difficulty[17]       6.99      7.57     5  1.77
## difficulty[18]       5.65      6.12     6  1.61
## difficulty[19]       6.19      6.75     8  2.44
## difficulty[20]       6.81      7.32     7  1.75
## difficulty[21]       7.10      7.61     5  1.71
## difficulty[22]       5.76      6.25     6  1.60
## difficulty[23]       6.74      7.29     6  1.69
## difficulty[24]       6.16      6.74     6  1.59
## difficulty[25]       8.53      9.04     6  1.76
## difficulty[26]       4.51      5.25     8  1.51
## difficulty[27]       8.63      9.04     5  1.70
## difficulty[28]       5.25      5.87     8  1.54
## difficulty[29]       7.24      7.71     7  1.65
## difficulty[30]       6.08      6.57     8  3.01
## difficulty[31]       7.03      7.48     6  1.66
## difficulty[32]       7.68      8.21     6  1.60
## difficulty[33]       8.80      9.43     6  1.70
## difficulty[34]       9.51     10.07     7  1.67
## difficulty[35]       6.75      7.18    11  1.27
## difficulty[36]       5.83      6.32     7  1.54
## difficulty[37]       8.72      9.20     7  1.76
## difficulty[38]       7.21      7.65     7  1.63
## difficulty[39]       6.93      7.38     6  1.64
## difficulty[40]       9.24      9.80     7  1.66
## difficulty[41]       7.02      7.58     7  1.60
## difficulty[42]       7.61      8.12     6  1.81
## difficulty[43]       8.53      8.98     6  1.80
## difficulty[44]       6.47      6.87     8  1.63
## difficulty[45]       9.04      9.51     6  1.73
## difficulty[46]       6.63      7.15     6  1.70
## difficulty[47]       8.87      9.45     6  1.72
## difficulty[48]       7.23      7.71     7  1.74
## difficulty[49]       7.93      8.38     6  1.64
## difficulty[50]       7.53      7.91     7  1.58
## difficulty[51]       7.56      8.11     6  1.63
## difficulty[52]       7.96      8.53     7  1.61
## difficulty[53]       7.30      7.77     7  1.69
## difficulty[54]       9.11      9.77     6  1.72
## difficulty[55]       7.78      8.30     6  1.79
## difficulty[56]       6.35      6.88     7  1.57
## difficulty[57]       6.98      7.50     6  1.64
## difficulty[58]       6.43      6.90    11  1.30
## difficulty[59]       6.61      7.16     7  1.63
## difficulty[60]       5.23      5.86    18  1.19
## difficulty[61]       7.65      8.08     7  1.59
## difficulty[62]       8.23      8.72     6  1.78
## difficulty[63]       8.29      8.83     6  1.86
## difficulty[64]       9.49     10.09     6  1.65
## difficulty[65]       8.09      8.63     6  1.78
## difficulty[66]       8.21      8.74     6  1.84
## difficulty[67]       7.00      7.54     7  1.65
## difficulty[68]       9.08      9.59     7  1.63
## difficulty[69]       5.91      6.39     7  1.59
## difficulty[70]       4.08      4.84    11  1.34
## difficulty[71]       5.94      6.46     7  1.64
## difficulty[72]       8.01      8.50     7  1.61
## difficulty[73]       6.99      7.44     6  1.69
## difficulty[74]       5.43      5.87    10  1.36
## difficulty[75]       6.21      6.68     7  1.60
## difficulty[76]       8.83      9.41     7  1.72
## difficulty[77]       7.46      7.92     7  1.65
## difficulty[78]       7.26      7.74     7  1.68
## difficulty[79]       8.14      8.66     7  1.59
## difficulty[80]       8.93      9.49     6  1.70
## difficulty[81]       7.49      7.97     6  1.73
## difficulty[82]       7.97      8.43     7  1.58
## difficulty[83]       8.58      9.07     7  1.64
## difficulty[84]       7.97      8.55     7  1.70
## difficulty[85]       7.54      8.04     6  1.66
## difficulty[86]       7.75      8.23     6  1.81
## difficulty[87]       6.77      7.34     7  1.63
## difficulty[88]       5.98      6.53    33  1.11
## difficulty[89]       7.59      8.13    11  1.25
## difficulty[90]       5.44      5.97     7  1.45
## difficulty[91]       7.78      8.34     6  1.66
## difficulty[92]       6.98      7.49     6  1.73
## difficulty[93]       7.83      8.35     5  1.75
## difficulty[94]       7.12      7.60     7  1.74
## difficulty[95]       7.13      7.74     6  2.60
## difficulty[96]       7.50      8.04     7  1.71
## difficulty[97]       7.27      7.82     5  1.70
## difficulty[98]       7.52      8.04     6  1.68
## difficulty[99]       5.99      6.44     8  2.37
## difficulty[100]      6.92      7.46     7  1.68
## difficulty[101]      7.88      8.32     7  1.58
## difficulty[102]      8.09      8.68     6  1.74
## difficulty[103]      8.46      8.93     6  1.63
## difficulty[104]      5.90      6.41     6  1.63
## difficulty[105]      7.43      7.92     7  1.55
## difficulty[106]      6.18      6.66     6  1.72
## difficulty[107]      8.02      8.56     6  1.75
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## difficulty[494]      7.79      8.35     7  1.59
## difficulty[495]      8.24      8.77     6  1.73
## difficulty[496]      8.25      8.82     6  1.69
## difficulty[497]      7.14      7.60     7  1.67
## difficulty[498]      7.50      8.02     6  1.68
## difficulty[499]      7.86      8.29     5  1.83
## difficulty[500]      8.04      8.42     7  1.62
## difficulty[501]     10.77     11.47     8  1.39
## difficulty[502]      7.41      7.98     6  1.74
## difficulty[503]      8.31      8.89     6  1.71
## difficulty[504]      6.04      6.56     8  1.60
## difficulty[505]      7.23      7.76     7  1.75
## difficulty[506]      8.08      8.62     6  1.71
## difficulty[507]      7.34      7.85     7  1.72
## difficulty[508]      8.41      9.04     6  1.69
## difficulty[509]      6.96      7.36     5  1.80
## difficulty[510]      8.19      8.70     6  1.68
## difficulty[511]      7.09      7.62     5  1.73
## difficulty[512]      7.54      8.00     6  1.70
## difficulty[513]      8.64      9.19     6  1.74
## difficulty[514]      8.00      8.54     6  1.69
## difficulty[515]      7.52      8.07     6  1.75
## difficulty[516]      6.40      6.92     6  1.57
## difficulty[517]      8.21      8.72     6  1.70
## difficulty[518]      8.32      8.95     6  1.75
## difficulty[519]      6.98      7.52     6  1.62
## difficulty[520]      7.91      8.43     5  1.84
## difficulty[521]      8.18      8.76     7  1.70
## difficulty[522]      8.08      8.48     6  1.74
## difficulty[523]      9.67     10.33     7  1.54
## difficulty[524]      8.71      9.20     7  1.67
## difficulty[525]      8.69      9.14     7  1.64
## difficulty[526]      8.34      8.87     6  1.68
## difficulty[527]      7.37      7.79     5  1.71
## difficulty[528]      8.02      8.53     6  1.70
## difficulty[529]      7.68      8.17     6  1.70
## difficulty[530]      8.60      9.11     6  1.72
## difficulty[531]      7.93      8.44     7  1.65
## difficulty[532]      8.00      8.51     6  1.74
## difficulty[533]      5.21      5.72     8  1.53
## difficulty[534]      7.59      8.18     6  1.67
## difficulty[535]      7.40      7.93     6  1.71
## difficulty[536]      9.43      9.94     7  1.57
## difficulty[537]      8.09      8.62     7  1.71
## difficulty[538]      9.16      9.74     6  1.64
## difficulty[539]      8.04      8.68     6  1.67
## difficulty[540]      8.80      9.27     6  1.75
## difficulty[541]      9.56     10.13     7  1.59
## difficulty[542]      8.54      9.13     7  1.61
## difficulty[543]      7.61      8.01     6  1.61
## difficulty[544]      9.15      9.68     7  1.73
## difficulty[545]      9.08      9.67     6  1.68
## difficulty[546]      7.77      8.28     5  1.80
## difficulty[547]      9.80     10.29     8  1.58
## difficulty[548]      7.01      7.53     6  1.65
## difficulty[549]      8.25      8.73     8  1.63
## difficulty[550]      7.81      8.27     6  1.61
## difficulty[551]      8.06      8.61     6  1.66
## difficulty[552]      8.19      8.72     6  1.66
## difficulty[553]      8.27      8.87     5  1.78
## difficulty[554]      8.02      8.51     6  1.61
## difficulty[555]      9.07      9.54     7  1.69
## difficulty[556]      7.76      8.28     7  1.63
## difficulty[557]      8.00      8.49     6  1.71
## difficulty[558]      7.43      7.89     7  1.72
## difficulty[559]      8.13      8.65     6  1.70
## difficulty[560]      9.40     10.18     6  1.70
## difficulty[561]      8.09      8.62     6  1.73
## difficulty[562]      6.94      7.44     6  1.79
## difficulty[563]      8.47      8.97     6  1.76
## difficulty[564]      8.63      9.17     7  1.64
## difficulty[565]      7.53      8.02     6  1.74
## difficulty[566]      6.87      7.42     6  1.67
## difficulty[567]      9.09      9.65     6  1.63
## difficulty[568]      9.83     10.52     7  1.50
## difficulty[569]      8.47      9.07     6  1.71
## difficulty[570]      8.51      9.00     6  1.67
## difficulty[571]      8.42      8.94     7  1.67
## difficulty[572]      7.48      7.92     6  1.71
## difficulty[573]      7.89      8.50     6  1.64
## difficulty[574]      9.59     10.23     6  1.73
## difficulty[575]      9.22      9.77     6  1.68
## difficulty[576]      8.90      9.40     7  1.64
## difficulty[577]      9.41      9.93     6  1.86
## difficulty[578]      9.90     10.48     7  1.63
## difficulty[579]      8.75      9.33     6  1.66
## difficulty[580]      9.28      9.85     7  1.64
## difficulty[581]      9.96     10.68     7  1.56
## difficulty[582]      9.28      9.87     6  1.68
## difficulty[583]      9.27      9.87     6  1.68
## difficulty[584]      7.72      8.19     7  1.58
## difficulty[585]      8.11      8.63     6  1.73
## difficulty[586]      6.68      7.15     6  1.70
## difficulty[587]      6.24      6.75     5  1.74
## difficulty[588]      7.61      8.01     6  1.73
## difficulty[589]      9.24      9.83     7  1.55
## difficulty[590]      9.67     10.35     7  1.53
## difficulty[591]      8.81      9.34     6  1.61
## difficulty[592]      7.26      7.76     7  1.72
## difficulty[593]     10.15     10.71     7  1.52
## difficulty[594]      9.42      9.98     6  1.76
## difficulty[595]      9.22      9.77     6  1.70
## difficulty[596]      9.66     10.27     6  1.68
## difficulty[597]      7.76      8.25     7  1.66
## difficulty[598]      9.18      9.79     7  1.67
## difficulty[599]      9.94     10.68     6  1.56
## difficulty[600]      9.39      9.98     8  1.55
## difficulty[601]      7.82      8.40     6  1.83
## difficulty[602]      8.90      9.47     6  1.59
## difficulty[603]     10.84     11.39    11  1.34
## difficulty[604]      9.70     10.36     6  1.66
## difficulty[605]      7.54      8.08     6  1.63
## difficulty[606]      9.64     10.06     7  1.56
## difficulty[607]      7.96      8.45     7  1.64
## difficulty[608]      9.87     10.50     6  1.59
## difficulty[609]      8.86      9.40     5  1.72
## difficulty[610]      8.71      9.26     6  1.71
## difficulty[611]     10.47     11.08     9  1.48
## difficulty[612]     10.46     10.87     7  1.58
## difficulty[613]      9.51     10.08     6  1.60
## difficulty[614]      8.76      9.24     7  1.71
## difficulty[615]      8.44      9.00     6  1.73
## difficulty[616]      8.15      8.62     5  1.81
## difficulty[617]      9.11      9.71     7  1.56
## difficulty[618]     10.10     10.68     7  1.56
## difficulty[619]      9.30      9.88     6  1.62
## difficulty[620]      6.34      6.81     7  1.68
## difficulty[621]      8.92      9.49     7  1.64
## difficulty[622]      7.70      8.18     6  1.75
## difficulty[623]      7.30      7.86     6  1.65
## difficulty[624]     10.04     10.71     6  1.47
## difficulty[625]      9.53     10.11     6  1.61
## difficulty[626]      9.74     10.31     7  1.54
## difficulty[627]      6.72      7.25     7  1.60
## difficulty[628]      6.84      7.38     7  1.60
## difficulty[629]      8.98      9.53     6  1.75
## difficulty[630]      7.12      7.63     7  1.57
## difficulty[631]      8.60      9.14     5  1.78
## difficulty[632]      8.00      8.54     7  1.65
## difficulty[633]      8.43      8.89     6  1.76
## difficulty[634]      8.13      8.73     5  1.68
## difficulty[635]      6.29      6.75     7  1.70
## difficulty[636]      8.78      9.40     7  1.71
## difficulty[637]      8.11      8.60     7  1.74
## difficulty[638]      8.08      8.53     7  1.70
## difficulty[639]      9.35      9.96     7  1.63
## difficulty[640]     10.90     11.70     7  1.40
## difficulty[641]      8.66      9.24     6  1.69
## difficulty[642]      9.29      9.87     6  1.67
## difficulty[643]     10.45     11.14     6  1.51
## difficulty[644]      9.76     10.43     8  1.63
## difficulty[645]      6.05      6.61    13  1.22
## difficulty[646]      9.37      9.87     7  1.65
## difficulty[647]      9.53     10.09     6  1.61
## difficulty[648]      8.86      9.40     6  1.69
## difficulty[649]      8.97      9.48     7  1.73
## difficulty[650]      9.22      9.86     5  1.76
## difficulty[651]      8.14      8.67     7  1.72
## difficulty[652]      8.55      9.17     7  1.63
## difficulty[653]      8.02      8.46     7  1.72
## difficulty[654]      9.14      9.75     6  1.60
## difficulty[655]      9.11      9.79     7  1.69
## difficulty[656]      9.53     10.12     7  1.63
## difficulty[657]      8.93      9.39     6  1.73
## difficulty[658]     10.41     10.98     8  1.53
## difficulty[659]      8.95      9.49     7  1.68
## difficulty[660]      7.70      8.18     6  1.64
## difficulty[661]     10.11     10.70     7  1.60
## difficulty[662]      8.08      8.56     5  1.73
## difficulty[663]      8.51      8.97     7  1.64
## difficulty[664]      9.32      9.81     6  1.61
## difficulty[665]      9.10      9.64     6  1.69
## difficulty[666]      8.51      9.05     6  1.71
## difficulty[667]      8.31      8.81     6  1.72
## difficulty[668]      8.65      9.15     7  1.70
## difficulty[669]      8.78      9.25     6  1.73
## difficulty[670]      7.59      8.11     7  1.62
## difficulty[671]      9.79     10.48     7  1.58
## difficulty[672]     10.22     10.83     9  1.49
## difficulty[673]      9.59     10.30     6  1.60
## difficulty[674]     10.86     11.43    11  1.42
## difficulty[675]      8.85      9.39     6  1.70
## difficulty[676]      8.89      9.48     6  1.69
## difficulty[677]      9.77     10.34     7  1.57
## difficulty[678]     10.38     11.00     9  1.44
## difficulty[679]      9.56     10.11     6  1.62
## difficulty[680]     10.04     10.63     6  1.69
## lp__            -29787.17 -29680.00     5  2.17
## 
## Samples were drawn using NUTS(diag_e) at Wed Aug  5 10:00:09 2015.
## For each parameter, n_eff is a crude measure of effective sample size,
## and Rhat is the potential scale reduction factor on split chains (at 
## convergence, Rhat=1).

Now re-merge.

coefs <- data.frame(summary(samps)$summary)
coefs$name <- rownames(coefs)

word_ids <- ws %>% 
  group_by(num_item_id) %>% 
  summarise(item = unique(item))

words <- coefs %>% 
  filter(str_detect(name, "difficulty")) %>%
  separate(name, c("name", "num_item_id"), "\\[") %>%
  mutate(num_item_id = as.numeric(str_replace(num_item_id, "]", ""))) %>%
  left_join(word_ids) %>%
  select(mean, se_mean, sd, item) %>%
  arrange(mean) %>%
  mutate(item = factor(item, 
                        levels = item,
                        labels = item))

and plot again:

qplot(item, mean, data=words[1:50,]) + 
  theme(axis.text.x = element_text(angle = 90, hjust = 1))

So clearly this model is getting us something. The only worry is that it’s still so slow.

Let’s try rewriting it so it is more vectorized? Hard to do under current Stan infrastructure.

Attempt #3: Binomials

But we can do an age aggregation and use binomials. (Since we’re not fitting subjects).

mcmurray_binom <-'
data {                            
  int<lower=1> W; // number of words
  int<lower=1> A; // number of ages
  int<lower=1> N; // number of datapoints (W*A)
  vector[A] age; // subject ages
  int produces[N]; // count data
  int attempts[N]; // count data
}

parameters {
  real<lower=0> mu_w; // word mean
  real<lower=0> mu_s; // subject mean
  real<lower=0> sigma_w; // word SD
  real<lower=0> difficulty[W]; // word mean
}

model {
  mu_w ~ normal(0, 10); 
  mu_s ~ normal(0, 10);
  sigma_w ~ normal(0, 5);
  difficulty ~ normal(mu_w, sigma_w);

  for (a in 1:A)
    for (w in 1:W) 
      produces[W*(a-1) + w] ~ binomial_logit(attempts[W*(a-1) + w], (mu_s * age[a]) - difficulty[w]);
}
'

Now run this with the full dataset. Interestingly, thinning is not recommended by many authors: see [http://doingbayesiandataanalysis.blogspot.ca/2011/11/thinning-to-reduce-autocorrelation.html]

n.words <- 680

complete <- ws %>%
  group_by(data_id) %>%
  summarize(complete = sum(!is.na(value)) == n.words) %>%
  filter(complete)


model.data <- ws %>%
  select(data_id, num_item_id, item, value, age) %>%
  filter(data_id %in% complete$data_id) %>%
  group_by(age, num_item_id) %>%
  summarise(n = sum(value), 
            N = length(value))

ages <- unique(model.data$age)

dat <- list(age = ages,
            produces = model.data$n, 
            attempts = model.data$N,             
            W = n.words, 
            A = length(ages), 
            N = n.words * length(ages))

ptm <- proc.time()
samps <- stan(model_code = mcmurray_binom, 
              cores = 4, 
              data = dat, iter = 300, warmup=100, thin=10, chains = 24, 
              pars = c("mu_w", "mu_s", "sigma_w", "difficulty")) 
## COMPILING THE C++ CODE FOR MODEL '5ce82d8c94af70eb89782a919fbc669f' NOW.
proc.time() - ptm
##    user  system elapsed 
##  32.402   2.807 204.931

Diagnostics.

traceplot(samps, pars = c('mu_w','mu_s','sigma_w'))

Success!

coefs <- data.frame(summary(samps)$summary)
coefs$name <- rownames(coefs)

word_ids <- ws %>% 
  group_by(num_item_id) %>% 
  summarise(item = unique(item))

words <- coefs %>% 
  filter(str_detect(name, "difficulty")) %>%
  separate(name, c("name", "num_item_id"), "\\[") %>%
  mutate(num_item_id = as.numeric(str_replace(num_item_id, "]", ""))) %>%
  left_join(word_ids) %>%
  select(mean, se_mean, sd, item) %>%
  arrange(mean) %>%
  mutate(item = factor(item, 
                        levels = item,
                        labels = item))

and plot again:

qplot(mean, as.numeric(item), 
      geom = "point", data=words) + 
  geom_text(aes(x = mean + .2, label = item), 
            data = words[seq(1,680, 10),], 
            size = 2)

Check the histogram:

qplot(mean, geom = "blank", data = words) + 
  geom_histogram(aes(y = ..density..)) + 
  geom_density(col = "red")

What are the words with means below 1?

filter(words, mean < 1)
##          mean    se_mean         sd           item
## 1  0.07866137 0.02970395 0.04227042          daddy
## 2  0.08430639 0.03116399 0.04425437          mommy
## 3  0.10447006 0.01989400 0.02834735           ball
## 4  0.10484346 0.02664622 0.03800157            bye
## 5  0.12863741 0.02971150 0.04219324          uh_oh
## 6  0.13190844 0.03823441 0.05421208             no
## 7  0.13629563 0.05586900 0.07915484           book
## 8  0.13650146 0.03106074 0.04407932            dog
## 9  0.15771244 0.05563103 0.07887128           baby
## 10 0.16074499 0.05847588 0.08304233             hi
## 11 0.17849331 0.07511224 0.10746221            car
## 12 0.18017045 0.02596099 0.03767650           shoe
## 13 0.18602319 0.07130439 0.10204358          juice
## 14 0.18850008 0.07338718 0.10457646            eye
## 15 0.20954523 0.02196423 0.03199990            moo
## 16 0.21262604 0.04603872 0.06738063           bird
## 17 0.21585838 0.07554811 0.10759685      woof_woof
## 18 0.22250346 0.02982616 0.04342450         banana
## 19 0.24575384 0.02708712 0.04170806           duck
## 20 0.25068177 0.10549353 0.15214774        balloon
## 21 0.27378803 0.03022874 0.04759371           meow
## 22 0.29671951 0.11898750 0.17312928        baa_baa
## 23 0.31312457 0.09542164 0.14351618           nose
## 24 0.35050920 0.13096858 0.19123260          apple
## 25 0.35677276 0.13592624 0.19878051           milk
## 26 0.37641563 0.18093183 0.26568744           ouch
## 27 0.38399311 0.06002686 0.09711413            hot
## 28 0.40662639 0.14753957 0.22004622         cheese
## 29 0.40687269 0.15173097 0.22780143     nightnight
## 30 0.46467848 0.20734438 0.30986546    quack_quack
## 31 0.52822860 0.23399173 0.35907858            hat
## 32 0.57959851 0.25838279 0.39580834          kitty
## 33 0.58218936 0.24439748 0.41509839        grandma
## 34 0.63385357 0.26639390 0.46131911           grrr
## 35 0.65212596 0.23823529 0.43062827         cookie
## 36 0.67479705 0.27047432 0.46438231            ear
## 37 0.75866744 0.39814320 0.60867500 water.beverage
## 38 0.79949471 0.53812639 0.78697826            cup
## 39 0.95883248 0.78482962 1.11847386        bubbles
## 40 0.96265875 0.47192226 0.71488456       peekaboo
## 41 0.97274645 0.76826542 1.09251239            yes
## 42 0.97636252 0.52863052 0.77498290           owie