Starting at a fixed time, we observe the gender of each newborn child at a certainhospital until a boy (B) is born. Let p 5 P(B), assume that successive births areindependent, and define the rv X by x 5 number of births observed. Thenp(1) 5 P(X 5 1) 5 P(B) 5 pp(2) 5 P(X 5 2) 5 P(GB) 5 P(G) ? P(B) 5 (1 2 p)pandp(3) 5 P(X 5 3) 5 P(GGB) 5 P(G) ? P(G) ? P(B) 5 (1 2 p)2pContinuing in this way, a general formula emerges:p(x) 5 5(1 2 p)x21p x 5 1, 2, 3,…0 otherwise(3.2)The parameter p can assume any value between 0 and 1. Expression (3.2) describesthe family of geometric distributions. In the gender scenario, p 5 .51 might beappropriate, but if we were looking for the first child with Rh-positive blood, then itmight be the case that p 5 .85.

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Starting at a fixed time, we observe the gender of each newborn child at a certain
hospital until a boy (B) is born. Let p 5 P(B), assume that successive births are
independent, and define the rv X by x 5 number of births observed. Then
p(1) 5 P(X 5 1) 5 P(B) 5 p
p(2) 5 P(X 5 2) 5 P(GB) 5 P(G) ? P(B) 5 (1 2 p)p
and
p(3) 5 P(X 5 3) 5 P(GGB) 5 P(G) ? P(G) ? P(B) 5 (1 2 p)2p
Continuing in this way, a general formula emerges:
p(x) 5 5(1 2 p)x21p x 5 1, 2, 3,…
0 otherwise
(3.2)
The parameter p can assume any value between 0 and 1. Expression (3.2) describes
the family of geometric distributions. In the gender scenario, p 5 .51 might be
appropriate, but if we were looking for the first child with Rh-positive blood, then it
might be the case that p 5 .85.

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