Unpaired t-tests : code in R
Delta is the ratio of group 2 to group 1, pow is the power, alpha is the (two-tailed) type I error, the difference between the means is mdiff with common sd. Group 1 is outputted. NoteL: the power.t.test R function only handles equal sized groups.
delta <- 1
pow <- 0.8
alpha <- 0.05
mdiff <- 2
sd <- 4
fn <- function(n1) {
(pow - (1 - pt(abs(qt(alpha/2,n1+(delta*n1)-2)), n1+(delta*n1)-2,
mdiff/sqrt((sd*sd)/n1+(sd*sd)/(n1*delta)))) )^2
}
nout<- nlm(fn,2)
nout<- trunc(nout$estimate+1)
print(nout)