[R] multiple t-tests across similar variable names

[arun]

HI,
May be this helps you.
set.seed(1)
dat2<-data.frame(apple_pre=sample(10:20,5,replace=TRUE),banana_pre=sample(25:35,5,replace=TRUE),apple_post=sample(20:30,5,replace=TRUE),banana_post=sample(40:50,5,replace=TRUE))
list2<-list(dat2[regmatches(colnames(dat2),regexpr("apple.*",colnames(dat2)))],dat2[regmatches(colnames(dat2),regexpr("banana.*",colnames(dat2)))])
res2<-do.call(rbind,lapply(lapply(list2,function(x) t.test(x[,1],x[,2],paired=TRUE)),function(x) data.frame(meandifference=x$estimate,CIlow=unlist(x$conf.int)[1],CIhigh=unlist(x$conf.int)[2],p.value=x$p.value)))
row.names(res2)<-unlist(unique(lapply(strsplit(colnames(dat1),"_"),`[`,1)))
res2
#       meandifference     CIlow    CIhigh     p.value
#apple            -9.8 -15.02385 -4.576150 0.006477650
#banana          -15.4 -21.64546 -9.154541 0.002382261

A.K.




----- Original Message -----
From: "Nundy, Shantanu" <snundy at chicagobooth.edu>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Wednesday, October 10, 2012 7:09 PM
Subject: Re: [R] multiple t-tests across similar variable names

Hi everyone-

I have a dataset with multiple "pre" and "post" variables I want to compare. The variables are named "apple_pre" or "pre_banana" with the corresponding post variables named "apple_post" or "post_banana". The variables are in no particular order.

apple_pre orange_pre orange_post pre_banana apple_post post_banana
person_1
person_2
person_3
...
person_x


How do I:
1. Run a series of paired t-tests for the apple_pre variables and pre_banana variables? Would be great to do something like ttest(*.*pre*.*,*.*post*.*).
2. Print the results from these t-tests in a table with col 1=mean difference, col 2= 95% conf interval, col 3=p-value.

Thank you kindly,
-Shantanu

Shantanu Nundy, M.D.
University of Chicago

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