within_group_mean: Difference in means within groups
In statlab/permuter: permuter: Permutation Tests

Compute difference in mean residual between treated and control within each group

1	within_group_mean(group, response, stratum, groups, strata)

`group`	Vector of group memberships or treatment conditions
`response`	Vector of measured outcomes, same length as group
`stratum`	Vector of stratum assignments, same length as group
`groups`	Vector of unique group labels
`strata`	Vector of unique stratum labels