On 05/01/2011 05:28 AM, Kevin Burnham wrote:
HI All,
I have a long data file generated from a minimal pair test that I gave to
learners of Arabic before and after a phonetic training regime. For each of
thirty some subjects there are 800 rows of data, from each of 400 items at
pre and posttest. For each item the subject got correct, there is a 'C' in
the column 'Correct'. The line:
tapply(ALLDATA$Correct, ALLDATA$Subject, function(x)sum(x=="C"))
gives me the sum of correct answers for each subject.
However, I would like to have that sum separated by Time (pre or post). Is
there a simple way to do that?
What if I further wish to separate by Group (T or C)?
Hi Kevin,
When I looked at this, I immediately thought of the brkdnNest function
(which uses tapply internally). In order to get the counts with the
current function, I had to create a new variable (newcorrect). However,
the idea so attracted me that I programmed it into the code (thanks).
Here is a way to get your summary by Subject and Time:
ALLDATA<-data.frame(Subject=rep(1:30,each=800),
Occasion=factor(rep(c("pre","post"),2400),levels=c("pre","post")),
Correct=sample(c("C","I"),2400,TRUE))
tapply(ALLDATA$Correct,list(ALLDATA$Subject,ALLDATA$Occasion),
function(x) sum(x=="C"))
library(plotrix)
brkdnNest(Correct~Subject+Occasion,ALLDATA,FUN="propbrk",trueval="C")
ALLDATA$newcorrect<-ALLDATA$Correct=="C"
brkdnNest(newcorrect~Subject+Occasion,ALLDATA,FUN="sum")
To get the three level breakdown, add another factor:
ALLDATA$Group<-rep(c("T","C"),each=1200)
brkdnNest(newcorrect~Group+Subject+Occasion,ALLDATA,FUN="sum")
Notice that this gives you all of the subjects for each Group, even if
they weren't in that Group. I'll work on that one, for I have just
switched to using "tapply" for this breakdown, as it doesn't discard NA
values (the cause of the minor bug in barNest)
Jim
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