 In this case, it can be shown that the number of people required to reach the 50% threshold is 23 "or fewer".For example, if half the people were born on one day and the other half on another day, then any "two" people would have a 50% chance of sharing a birthday.It may well seem surprising that a group of just 23 individuals is required to reach a probability of 50% that at least two individuals in the group have the same birthday