[R] multiple t-tests

B Nichols bjnicho2 at mail.usf.edu
Sun Aug 17 21:27:44 CEST 2008

This is probably a basic data manipulation issue and an easy answer, but 
I can't seem to make it work. I am more used to working with data by 
participant, not by answer.

I have survey data, one answer per line (18000+ lines and counting), and 
each line includes variables for:

prepost (0 for pre, 1 for post)

question (the number of the question, from S1 to S85)

answer (a Likert range of 0-4)

I want to do t.tests (and Wilcox tests actually) on each question's pre 
and post means, but I can't figure out a single statement that will do 
this automatically, and I don't want to have to make 85 individual calls.

An individual call that seems to work with either t.test or wilcox.test 
looks like:

t.test(subset(answer, question=="S04") ~ subset(prepost, question=="S04"))

It would also be great to have a table including the difference between 
pre and posttest means. It seems these things should be doable, but I 
just can't find the right statement with the data in this format.



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