Stratified sampling. A city has n blocks of which n, have x, inhabit- ants each (n + n + n). Let mn,x,/n be the mean number of inhabitants per block and put a = n na - m. In sampling without replacement blocks are selected at random, and in each the inhabitants are counted. Let X, . . ., X, be the respective number of inhabitants. Show that ar(n-r) E(X1 + + X,) = mr Var (X1 + + X,) n-1 (In sampling with replacement the variance would be larger, namely, ar.)