Describe FAQ/intsg here.
From Boniface (p.42-3)
SS(subjects x group) = $$ \sum_text{ij} (X_text{ij} - (\bar{X}_text{group} - \bar{X}) - \bar{X}_text{sub})^text{2} $$
= $$ \sum_text{ij} (X_text{ij} - \bar{X}_text{group} - \bar{X}_text{sub} + \bar{X})^text{2} $$
MS(error) = SS(subjects x group) / ((Nsub-1)(Ngroup-1))
The (uncorrected) t statistic on 2n-2 degrees of freedom differences between a pair of means is
$$\frac{\mbox{difference in means}{\frac{\mbox{2MS(error)}{\mbox{n}}}$$