Page "Birthday problem" Paragraph 5

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.