Welsh test by permutations

November 1, 2009

(This article was first published on [R] tricks, and kindly contributed to R-bloggers)

WelshPerm <- function(response,variable,nperm=999,...){

base <- oneway.test(response~variable,...)

base.p <- base$p.value
base.W 	<- base$statistic

count <- 1

# Permutation loop
for(i in 1:nperm){

	SAMPLE <- sample(response)

	welsh.perm<-oneway.test(SAMPLE~variable,...)
	welsh.perm.p<-welsh.perm$p.value
	welsh.perm.W<-welsh.perm$statistic 	if(abs(welsh.perm.W) >= abs(base.W)) {count <- count+1}
}
result=count/(nperm+1)
return(result)
}