Performs all pairwise comparisons between group means, testing for significance using Scheffe's F-test. Returns an upper-triangular table in a format described in the manual. Also see the function `print-scheffe-table.' `Group-means' and `group-sizes' should be sequences. The arguments `ms-error' and `df-error' are the mean square error within groups and its degrees of freedom, both of which are computed by the analysis of variance. An ANOVA test should always be run first, to see if there are any significant differences.