February 20, 2013 Class Notes

Reviewing the car price models.

cars1 = fetchGoogle("https://docs.google.com/spreadsheet/pub?key=0Akkdc3QLDbyAdGxNRDAzcG5aRGJBSVFHa3ljaHVObnc&output=csv")

## Loading required package: RCurl

## Loading required package: bitops

confint(lm(Price ~ Age, data = cars1))

##             2.5 % 97.5 %
## (Intercept) 20312  48002
## Age         -4309  -1023

confint(lm(Price ~ Miles, data = cars1))

##                 2.5 %     97.5 %
## (Intercept) 19382.186  2.696e+04
## Miles          -0.123 -6.394e-02

confint(lm(Price ~ Age + Miles, data = cars1))

##                  2.5 %     97.5 %
## (Intercept) 22949.5370  3.886e+04
## Age         -2289.1924 -7.577e+01
## Miles          -0.1064 -4.523e-02

Construct models using Zip, Age, Miles and discuss the units. What happens with interaction terms? What are the units of those?

prius = fetchData("M155/Spring2013/ToyotaPrius.csv")

## Retrieving from
## http://www.mosaic-web.org/go/datasets/M155/Spring2013/ToyotaPrius.csv

lm(Price ~ Age, data = prius)

## Error: object 'Age' not found

What does the intercept mean? Price of a hypothetical prius produced in year 0 — the Roman prius (not to be confused with Priapus).

prius = transform(prius, Age = 2013 - Year)
mod = lm(Price ~ Age + Miles, data = prius)
mod

## 
## Call:
## lm(formula = Price ~ Age + Miles, data = prius)
## 
## Coefficients:
## (Intercept)          Age        Miles  
##    2.64e+04    -1.78e+03    -1.71e-02

# make a 2-d plot of this
f1 = makeFun(mod)
plotFun(f(Age = a, Miles = m) ~ a & m, a.lim = c(0, 10), m.lim = c(0, 1e+05))

## Error: could not find function "f"

lm(Price ~ Age + Miles + Zip, data = prius)

## 
## Call:
## lm(formula = Price ~ Age + Miles + Zip, data = prius)
## 
## Coefficients:
## (Intercept)          Age        Miles    Zipz55347    Zipz78717  
##    2.62e+04    -1.85e+03    -1.29e-02    -5.94e+02     1.50e+01  
##   Zipz90210  
##    1.63e+03

lm(Price ~ Age + Miles * Zip, data = prius)

## 
## Call:
## lm(formula = Price ~ Age + Miles * Zip, data = prius)
## 
## Coefficients:
##     (Intercept)              Age            Miles        Zipz55347  
##        2.45e+04        -1.64e+03         1.40e-03         1.85e+03  
##       Zipz78717        Zipz90210  Miles:Zipz55347  Miles:Zipz78717  
##        2.31e+03         3.73e+03        -4.58e-02        -4.41e-02  
## Miles:Zipz90210  
##       -4.31e-02

Watch out! An outlier in price.

mod = lm(Price ~ Age * Miles, data = prius)
f = makeFun(mod)
plotFun(f(Age = a, Miles = m) ~ a & m, a.lim = c(0, 10), m.lim = c(0, 1e+05))

plot of chunk unnamed-chunk-5 

max(Miles, data = prius)

## [1] 328000

pp = subset(prius, Miles < 1e+05)
mod = lm(Price ~ Age * Miles, data = pp)
f = makeFun(mod)
plotFun(f(Age = a, Miles = m) ~ a & m, a.lim = c(0, 10), m.lim = c(0, 1e+05))

plot of chunk unnamed-chunk-5

Untangling

Untangling