Sampling Subgroups from Data

This is a brief tutorial on how to subsample groups from a data frame using dplyr.

library(devtools)
install_github("jofrhwld/UhUm", quiet = T)
library(UhUm)
library(dplyr)
library(ggplot2)

um_PNC has every token of UH and UM from a collection of recorded speech called the Philadelphia Neighborhood Corpus (PNC). idstring corresponds to a unique id for each speaker.

  head(um_PNC)

##    idstring word start_time end_time vowel_start vowel_end nasal_start
## 1 PH00-1-1-   UH     24.388   24.688      24.388    24.688          NA
## 2 PH00-1-1-   UH     34.963   35.243      34.963    35.243          NA
## 3 PH00-1-1-   UM     37.903   38.273      37.903    38.123      38.123
## 4 PH00-1-1-   UH     44.453   44.653      44.453    44.653          NA
## 5 PH00-1-1-   UH     57.648   57.828      57.648    57.828          NA
## 6 PH00-1-1-   UH     62.298   62.487      62.298    62.487          NA
##   nasal_end next_seg next_seg_start next_seg_end chunk_start chunk_end
## 1        NA        S         24.688       24.867      24.388    25.288
## 2        NA        F         35.243       35.353      34.963    37.113
## 3    38.273       sp         38.273       38.393      37.903    38.803
## 4        NA       DH         44.653       44.682      44.453    45.623
## 5        NA      AY1         57.828       57.878      57.648    58.968
## 6        NA       sp         62.487       62.978      62.068    62.487
##   nwords sex year age ethnicity schooling transcribed    total nvowels
## 1   6551   m 2000  21       i/r        14    2810.733 2814.345    3078
## 2   6551   m 2000  21       i/r        14    2810.733 2814.345    3078
## 3   6551   m 2000  21       i/r        14    2810.733 2814.345    3078
## 4   6551   m 2000  21       i/r        14    2810.733 2814.345    3078
## 5   6551   m 2000  21       i/r        14    2810.733 2814.345    3078
## 6   6551   m 2000  21       i/r        14    2810.733 2814.345    3078

  um_PNC %>%
      group_by(idstring)%>%
      tally()%>%
      ggplot(aes(n))+
        geom_bar()

## stat_bin: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.

Subsampling

If you wanted to examine just random sample of 10 speakers from this whole data frame, the steps are:

• define the groups that will be sampled
• summarise, in any way (here, I’ll use tally() %>% select(-n))
• sample from the summary using sample_n or sample_frac
• left_join the sample with the original data frame.

  subsamp_10 <- um_PNC %>%
                  group_by(idstring) %>%  
                  tally() %>% select(-n) %>%
                  sample_n(size = 10, replace = F)%>%
                  left_join(um_PNC)

## Joining by: "idstring"

  subsamp_10 %>%
    group_by(idstring, sex)%>%
    tally()%>%
    ggplot(aes(idstring, n))+
      geom_bar(stat = 'identity')+
      facet_grid(.~sex, scales = "free_x", space = "free_x")

Balanced across subgroups

If you want to ensure an equal number of men and women in the subsample, group by sex as well.

  subsamp_10_balanced <-  um_PNC%>%
                                group_by(sex, idstring)%>%
                                tally() %>% select(-n)%>%
                                sample_n(size = 5, replace = F)%>%
                                left_join(um_PNC)

## Joining by: c("sex", "idstring")

  subsamp_10_balanced %>%
    group_by(idstring, sex)%>%
    tally()%>%
    ggplot(aes(idstring, n))+
      geom_bar(stat = 'identity')+
      facet_grid(.~sex, scales = "free_x", space = "free_x")

Proportional subgroups

If you want the sub sample to have the same gender ratio as the full sample, use sample_frac

  subsamp_prop <- um_PNC %>%
                    group_by(sex, idstring)%>%
                    tally() %>% select(-n) %>%
                    sample_frac(size = 0.03, replace = F) %>%
                    left_join(um_PNC)

## Joining by: c("sex", "idstring")

  subsamp_prop %>%
    group_by(idstring, sex)%>%
    tally()%>%
    ggplot(aes(idstring, n))+
      geom_bar(stat = 'identity')+
      facet_grid(.~sex, scales = "free_x", space = "free_x")