1 Session 03: Linear modeling 2

Discussion of SFLWR3, Section 5.5

1.1 Class activities

1.1.1 A multifactorial model selection process

Does the reaction time to a word (in ms) vary as a function of

  • the Zipf frequency of that word (ZIPFFREQ);
  • the language that word was presented in (LANGUAGE: English vs. Spanish);
  • the speaker group that words was presented to (GROUP: English vs. heritage);
  • any pairwise interaction of these predictors;
  • the three-way interaction of these predictors?
rm(list=ls(all.names=TRUE))
library(car); library(effects)
summary(x <- read.delim(   # summarize x, the result of loading
   file="105_02-03_RTs.csv",  # this file
   stringsAsFactors=TRUE)) # change categorical variables into factors
##       CASE            RT             LENGTH      LANGUAGE        GROUP
##  Min.   :   1   Min.   : 271.0   Min.   :3.000   eng:4023   english :3961
##  1st Qu.:2150   1st Qu.: 505.0   1st Qu.:4.000   spa:3977   heritage:4039
##  Median :4310   Median : 595.0   Median :5.000
##  Mean   :4303   Mean   : 661.5   Mean   :5.198
##  3rd Qu.:6450   3rd Qu.: 732.0   3rd Qu.:6.000
##  Max.   :8610   Max.   :4130.0   Max.   :9.000
##                 NA's   :248
##       CORRECT        FREQPMW           ZIPFFREQ     SITE
##  correct  :7749   Min.   :   1.00   Min.   :3.000   a:2403
##  incorrect: 251   1st Qu.:  17.00   1st Qu.:4.230   b:2815
##                   Median :  42.00   Median :4.623   c:2782
##                   Mean   :  81.14   Mean   :4.591
##                   3rd Qu.: 101.00   3rd Qu.:5.004
##                   Max.   :1152.00   Max.   :6.061
##

Some exploration of the relevant variables:

# the predictor(s)/response on its/their own
hist(x$RT); hist(x$RT, breaks="FD")

hist(x$ZIPFFREQ); hist(x$ZIPFFREQ, breaks="FD")

table(x$LANGUAGE)
##
##  eng  spa
## 4023 3977
table(x$GROUP)
##
##  english heritage
##     3961     4039
# the predictor(s) w/ the response
plot(
   main="RT in ms as a function of frequency per million words",
   pch=16, col="#00000030",
   xlab="Zipf frequency", xlim=c(  3,    6), x=x$ZIPFFREQ,
   ylab="RT (in ms)"    , ylim=c(250, 4250), y=x$RT); grid()
abline(lm(x$RT ~ x$ZIPFFREQ), col="red", lwd=2)