temporary just to hold formulas
formulas should also be correct for unweighted means
not sure what happens if we have several variables. everything should go through vectorized but not checked yet.
extend to any number of groups or write a version that works in that case, like in SAS and SPSS.
Methods
test_equal_var() | Levene test for independence |
ttest_ind([alternative, usevar]) | ttest for the null hypothesis of identical means |