# The Birthday Problem: A short lesson in probability.

*Last revised 8/24/05*

## Introduction with simulation

**Happy Birthday!** There's a birthday in your class today! Or
will there be two? How likely is it that two people in your class
have the same birthday? Say your class has 28 students.

There are a number of ways to approach this problem. The most
common is to take a survey and see if it happens that two birthdays
fall on the same day. But if it happens in the surveyed class, will
it occur in another class with different students? The question of
how likely it is for any given class is still unanswered.

Another way is to survey more and more classes to get an idea of
how often the match would occur. This can be time consuming and may
require a lot of work. But a computer can help out. Below is a
simulation of the birthday problem. It will generate a random list
of birthdays time after time. Simply type the number of people in
your virtual class into the textbox and hit
**<ENTER>** to run the experiment.

Choose a number for your class size and do
10 trials with that size.

Click to see a graphical version of this applet

Applet Source, Written by Nicholas Exner.

Now what do you think the probability of a match is?

It may surprise you that there were so many matches. Let's look at
an explanation for this
problem.