In SAS proc power, there is an example for Determining Required Sample Size for a Two-Sample t Test. Here is the original code from SAS proc power;    twosamplemeans       groupmeans   = (13 14) (13 14.5) (13 15)       stddev       = 1.2 1.7       groupweights = 1 | 1 2 3       power        = 0.9       ntotal       = .; run; I calculate the sample size by using formula, the result is always smaller the one obtained from SAS output. For example, the first sample size I got is 60.466176 but SAS got 62.507429. Basically,  the fractional n total from SAS is always bigger then the one I calculated based on the common formula which is n=4*stddev^2*(1.96+1.28)^2/(14-13). I want to know how SAS compute the sample size and base on what formula. I hope someone can help me to find it out. Thanks. ... View more