> #http://cran.r-project.org/ AND DOWNLOAD THE R LATEST RELEASE: R-2.x.y.
> 
> #COPY AND PASTE INTO R AND, AMEND INPUTS AS DESIRED] 
> 
> 
> delta <- 1
> pow <- 0.9
> alpha <- 0.05
> mdiff <- 3
> sd <- 4
> #
> #[THEN COPY AND PASTE THE BELOW INTO R] 
> #[SAMPLE SIZE FOR GIVEN POWER]
> 
> 
> 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)
> cat("Sample size per group for (two-tailed) unpaired t-test")
Sample size per group for (two-tailed) unpaired t-test> print(nout)
[1] 39
> 
> 
> #[COPY AND PASTE INTO R]
> #[POWER FOR A GIVEN SET OF CRITERIA] 
> 
> 
> fn <- function (alpha,delta,mean1,mean2,sd1,sd2,n1) {
+ mdiff <- mean1 - mean2
+ sd <- sqrt((n1*sd1*sd1+n1*delta*sd2*sd2)/(n1+delta*n1-2))
+ pow <- 1 - pt(abs(qt(alpha/2, n1+(delta*n1)-2)), n1+(delta*n1)-2,
+ mdiff/sqrt((sd*sd)/n1+(sd*sd)/(n1*delta)))
+ cat("Power for (two-tailed) unequal sized groups unpaired t-test =")
+ print(pow)
+  }
> #
> #[THEN ENTER INPUTS IN FORM BELOW] 
> #
> 
> delta <- 1
> alpha <- 0.05
> mean1 <- 3
> mean2 <- 0
> sd1 <- 4
> sd2 <- 4
> n1 <- 38
> #
> #AND RUN 
> #
> fn(alpha,delta,mean1,mean2,sd1,sd2,n1)
Power for (two-tailed) unequal sized groups unpaired t-test =[1] 0.8894802
> 
