Birthday Fun, Part I


Recently my friend Jibreel and his wife had their second child--exactly two years to the day that they had their first child.  This got me thinking about the probability that such an event occurs.   The answer can be calculated surprisingly simply if you make an assumption or two that simplifies the math.

In a prior post, We All Scream for Ice Cream, I spoke about probability--and the notion of permutations and combinations.  These concepts will be somewhat important here as well.  

First, let's assume we know when the first child is born.   There are 365 days in a year, so the odds of the second child being born on the exact same day is simply 1/365.  

Now, there are a number of simplifying assumptions embedded here-- for example, that the likelihood of being born on any day in the year is the same and that the actions of the parents has nothing to do with the birthday.

There is an entire forum on this question on quora which delves into this issue even more.

And here is a news article about a family who had 4 kids on the same day--can you calculate the probability of this event?

Coming soon, I will have a second blog with another popular birthday problem attributed to Von Mises--how many people are required to be in a room to find two people with the same birthday?