Understanding The Problem
The birthday problem asks whether any of the people in a given group has a birthday matching any of the others — not one in particular. (See "Same birthday as you" below for an analysis of this much less surprising alternative problem.)
In the example given earlier, a list of 23 people, comparing the birthday of the first person on the list to the others allows 22 chances for a matching birthday, the second person on the list to the others allows 21 chances for a matching birthday, third person has 20 chances, and so on. Hence total chances are: 22+21+20+....+1 = 253, so comparing every person to all of the others allows 253 distinct chances (combinations): in a group of 23 people there are pairs.
Presuming all birthdays are equally probable, the probability of a given birthday for a person chosen from the entire population at random is 1/365 (ignoring Leap Day, February 29). Although the pairings in a group of 23 people are not statistically equivalent to 253 pairs chosen independently, the birthday paradox becomes less surprising if a group is thought of in terms of the number of possible pairs, rather than as the number of individuals.
Read more about this topic: Birthday Problem
Famous quotes containing the word problem:
“Great speeches have always had great soundbites. The problem now is that the young technicians who put together speeches are paying attention only to the soundbite, not to the text as a whole, not realizing that all great soundbites happen by accident, which is to say, all great soundbites are yielded up inevitably, as part of the natural expression of the text. They are part of the tapestry, they aren’t a little flower somebody sewed on.”
—Peggy Noonan (b. 1950)