Starting at a fixed time, we observe the gender of each newborn child at a certain hospital until a boy (8) is born. Let p = P{B), assume that successive births are independent, and define the rv X by X = number of births observed. Then P(1) = P(X = 1) = P(B) = p(2) = P(X = 2) = P(GB) = P(G) middot P(B) = and p(3) = P(X = 3) = P(GGB) = P(G) middot P(G) middot P(B) = Continuing in this way, a general formula emerges: P(x) = {(l -P)_0^x-1 P x = 1, 2, 3, ... otherwise The parameter p can assume any value between and The expression above describes the family of geometric distributions. In the gender example, p = 0.51 might be appropriate, but if we were looking for the first child with Rh-positive blood, then we might have p = 0.85.