121. The article “Three Sisters Give Birth on the Same Day”

(Chance, Spring 2001, 23–25) used the fact that three Utah sisters had all given birth on March 11, 1998 as a basis for posing some interesting questions regarding birth coincidences.

a. Disregarding leap year and assuming that the other 365 days are equally likely, what is the probability that three randomly selected births all occur on March 11?

Be sure to indicate what, if any, extra assumptions you are making.

b. With the assumptions used in part (a), what is the probability that three randomly selected births all occur on the same day?

c. The author suggested that, based on extensive data, the length of gestation (time between conception and birth)

could be modeled as having a normal distribution with mean value 280 days and standard deviation 19.88 days.

The due dates for the three Utah sisters were March 15, April 1, and April 4, respectively. Assuming that all three due dates are at the mean of the distribution, what is the probability that all births occurred on March 11?

[Hint: The deviation of birth date from due date is normally distributed with mean 0.]

d. Explain how you would use the information in part (c)

to calculate the probability of a common birth date.