trouble using sapply to perform multiple t-test
On 02/16/2014 07:16 AM, David Romano wrote:
Hi folks, I'm having trouble with code that used to work, and I can't figure out what's going wrong. I'd be grateful for any help in sorting this out. Suppose I define a matrix
mm<- matrix(1:15, 25,2)
and compare the first 15 values of column 1 of mm to the values remaining in the same column and obtain p values as follows:
c1<- mm[,1] out<- t.test(c1[1:15],c1[16:25]) ; out$p.value
This of course works fine, but if I try to embed this line in a call to sapply to repeat this for each column, I get the following:
mm.pvals<- sapply(mm, function(x) {out<- t.test(x[1:15],x[16:25]) ;
out$p.value}) Error in t.test.default(x[1:15], x[16:25]) : not enough 'x' observations What is baffling is code like this has worked for me before, and I can't tell what's triggering the error.
Hi David,
You have here the old "s" problem. I suspect that you previously ran
this code on a data frame, not a matrix. Try this:
mm.pvals<-apply(mm,2,
function(x) {out <- t.test(x[1:15],x[16:25]);out$p.value})
Jim