⇤ ← Revision 1 as of 2008-01-21 15:38:44
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= Unpaired t-tests : code in R = | = Sample size for unequal sized groups in an unpaired t-tests = |
Sample size for unequal sized groups in an unpaired t-tests
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)