Illinois State University Mathematics Department

MAT 312: Probability and Statistics for Middle School Teachers

Dr. Roger Day (

The Birth Day Problem

You may find it hard to believe that with only 23 people present, the probability of a birth day match is greater than 50%! Later in the course we'll calculate probabilities for matching birth days for various numbers of people. Our calculations will depend on the probability of a complementary event to at least one match in a group: The probability that none of those present share a common birth day.