For two groups (n1, x-bar1, $s1^2$) and (n2, x-bar2, $s2^2$): Combined mean x-bar = $\frac{n1*x-bar1 + n2*x-bar2}{(n1+n2)}$. Combined variance $s^2$ = [n1*($s1^2$ + $d1^2$) + n2*($s2^2$ + $d2^2$)]/(n1+n2), where d1 = x-bar1 - x-bar, d2 = x-bar2 - x-bar. The d-terms account for the difference between individual group means and the combined mean. If the two groups have the same mean, combined variance simplifies to the weighted average of variances.