ty in (b) when n = 1 million, 4 million, and 10 million.
17. A Time article (13 March 1989, Medicine section) sounded warning bells about the fast-
growing in vitro fertilization industry, which caters to infertile couples who are desperate
to have children. At the time, U.S. in vitro programs charged in the vicinity of $7,000 per
attempt. There is a lot of variability in the success rates both between clinics and within a
clinic over time, but we will use an average rate of 1 success in every 10 attempts.
Suppose that 4 attempts ($28,000) is the maximum a couple feels they are prepared
to pay and that they will try until they are successful up to that maximum. (See Case
Study 5.2.1.)
(a) Write a probability function for the number of attempts made.
(b) Compute the expected number of attempts made. Also compute the standard deviation
of the number of attempts made.
(c) What is the expected cost when embarking on this program?
(d) What is the probability of still being childless after paying $28,000?
The calculations you have made assume that the probability of success is always the same
at 10% for every attempt. This will not be true. Quoted success rates will be averaged over
both couples and attempts. Suppose the population of people attending the clinics is made
up of three groups. Imagine that 30% is composed of those who will get pregnant compar-
atively easily, say pr(success) = 0.2 per attempt; 30% are average with pr(success) = 0.1
per attempt; and the third group of 40% have real difficulty and have pr(success) = 0.01.13
After a couple are successful, they drop out of the program. Now start with a large number,
say 100,000 people and perform the following calculations.
?(e) Calculate the number in each group you expect to conceive on the first attempt and
hence the number in each group who make a second attempt.
Note: It is possible to find the probabilities of conceiving on the first try, on the second given failure
at the first try, and so on, using the methods of Chapter 4. You might even like to try it. The method
you were led through here is much simpler, though informal.
?(f) Find the number in each group getting pregnant on the second attempt and hence
the number in each group who make a third attempt. Repeat for the third and fourth
attempts.