/*** * H H * H H * HHHH aa ppp ppp y y * H H a a p p p p y y * H H aaa ppp ppp yyy * p p y * p p yyy * * BBBB t h d * B B ii t h d * BBBB rrr ttt hhh ddd aa y y * B B ii r t h h d d a a y y * BBBB ii r tt h h ddd aaa yyy * y * yyy */
Today, in a not-so-veiled attempt to have everyone get more familiar with their classmates, I had everyone gather names and birthdays of everyone in each class and we looked for matching birthdays.
I prefaced this with these two questions about birthdays:
- What is the probability that at least two people in this class have the same birthday?
- How big a class would be required for that probability to reach 99%?
The answers are better than 50% for the first, with a class size of at least 23. For the second, surprisingly, it is only 57. (100% would require 366, of course.)
This is a classic problem in probability. Here’s a graph of the probabilities compared to the group size:
Extra credit for anyone who gives an explanation of the math behind this to the class.
We found 1, 3, and zero matches in periods 1, 2, and 3, respectively.