= Equal samples t test for a given power in R =

For a two-tailed test with type I error, alp, with inputted mean and average group sd will have power equal to pow. 

[TYPE INTO R THE DESIRED INPUTS ALPHA, MEAN, SD AND N USING VALUES IN FORM BELOW].

The R syntax below 

{{{
pow <- 0.8
beta <- 0.05
mdiff <- 2
sd <- 4
}}}
the the below will give the equal samples in ''nout''. 
{{{
fn <- function(n) {
(pow - (1 - pt(abs(qt(beta/2,(2*n)-2)),(2*n)-2,mdiff/sqrt(2*sd*sd/n))) )^2
 }

nout<- nlm(fn,2)
nout<- trunc(nout$estimate+1)
print(nout)
}}}

This can be run in SPSS 16 using R integrator plug-in (see [[FAQ/powspsinR| here)]].