Effect size (delta) is specified by the difference between the largest mean and the smallest mean, in units of the within-cell standard deviation ( sigma = the square root of the MSE):
largest mean - smallest mean delta = ---------------------------- sigmaThe minimum power specification corresponds to the alternative hypothesis that all means other than the two extreme one are equal to the grand mean. The computations assume: (a) fixed effects, and (b) equal sample sizes in all treatments. Under these assumptions, the non-centrality parameter of the F-distribution can be calculated as N (delta^2)/2, where N is the sample size per treatment.
Effect size delta values are typically in the range of 0 - 3. In social science applications, values of delta = 0.25, 0.75, and 1.25 or greater correspond to "small", "medium", and "large" effects, according to Cohen & Cohen, Statistical Power Analysis for the Behavioral Sciences.
However, the program can also be used for ANY fixed effect in ANY crossed factorial design, by designating the levels of the effect of interest as 'A', and the levels of all other crossed factors as 'B'.