The R code below may be run in a syntax window in SPSS 16 after using the R integrator plug-in. You need to enter the inputs (Type I error, difference in means, sd and required power) in the BEGIN DATA sandwich. Sample size (in both groups) is outputted in the SPSS output window.
DATA LIST free
/alpha (f10.5) mdiff (f10.5) sd (f10.5) power (f10.5).
BEGIN DATA.
0.05 6.0 10.0 0.90
END DATA.
begin program r.
casedata <- spssdata.GetDataFromSPSS()
beta <- casedata$alpha
mdiff <- casedata$mdiff
sd <- casedata$sd
pow <-casedata$power
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)
outp <- data.frame(A=c(nout,beta,mdiff,sd,pow),row.names=c("N","alpha","Mean Diff","sd","Power"))
spsspivottable.Display(outp,
title="Sample Size needed", rowdim=" ", format=formatSpec.Correlation)
end program.