subset.xbal: Variable, group interactions

[markmfredrickson]

The subset.xbal function takes a naive approach to selecting variables and groups. Variable selection only changes the $results array and group selection only applies to the $groups array. A proper implementation should interact the two, such that when selecting variables, only groups that include those variables are returned. Likewise, when selecting groups, only the variables in those groups are returned.

For example

xb <- xBalance(Z ~ X + Y + Z, data = d, groups = ("XY" = c("X", "Y")))
dimnames(select(xb, groups = c("XY"))$results)[[1]]

Should be c("X", "Y") (i.e., the variable Z is dropped).