I'd look at it like this. So now we have: set.seed(12345) treatment = c(rep(0, 4), rep(1, 4)) gender1 = rep(c(1, 0), 4) gender2 = rep(c(0, 1), 4)

I'd look at it like this. So now we have: set.seed(12345) treatment = c(rep(0, 4), rep(1, 4)) gender1 = rep(c(1, 0), 4) gender2 = rep(c(0, 1), 4) gender3 = rep(c(.5, -.5), 4) result = rnorm(8) summary(lm(result ~ treatment*gender1)) summary(lm(result ~ treatment*gender2)) summary(lm(result ~ treatment*gender3)) Each gives a different estimate of the treatment effect and the third also gives a different uncertainty than the others for that effect.