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The Birthday Problem – in a nutshell.

March 21, 2019

How large should a randomly chosen group of people be, to make it more likely than not that at least two of them share a birthday?

For convenience, assume that all dates in the calendar are equally likely as birthdays, and ignore the Leap Year special of February 29th

The first thing to look at is the likelihood that two randomly chosen people would share the same birthday.

Let’s call them Felix and Felicity. Say Felicity’s birthday is May 1st. What is the chance that Felix shares this birthday with Felicity? Well there are 365 days in the year, and only one of these is May 1st and we are assuming that all dates in the calendar are equally likely as birthdays. What we call the sample space is, therefore 365 days and each particular birthday is an ‘event’ in that sample space.

So, the probability that Felix’s birthday is May 1st is 1/365, and the chance he shares a birthday with Felicity is 1/365.

So what is the probability that Felix’s birthday is not May 1st? It is 364/365. This is the probability that Felix doesn’t share a birthday with Felicity.

More generally, for any randomly chosen group of two people, the probability that the second person has a different birthday to the first is 364/365.

With 3 people, the chance that all three are different is the chance that the first two are different (364/365) multiplied by the chance that the third birthday is different (363/365).

So, the probability that 3 people have different birthdays = 364/365 x 363/365

Now, suppose that the room contains four people. What is the probability that at least two of these people share the same birthday?

The probability that 4 people have different birthdays = 364 x 363 x 362 / 365 x 365 x 365

We can then subtract this probability from 1 to establish the probability that at least two of the four share a birthday.

Probability that none of the four people share the same birthday =

365 x 364 x 363 x 362 / 365 x 365 x 365 x 365 = 0.984

Probability that at least two of them share the same birthday = 1 – 0.984 = 0.016

Similarly, it can be calculated that the probability of at least two sharing a birthday increases as n, the number in the room, increases, as below:

n = 16; probability = 0.281

n= 23; probability = 0.505

n = 32; probability = 0.754

n = 40; probability = 0.892

So, the probability that two share a birthday exceeds 0.5 in a room of 23 or more people.

So how large should a randomly chosen group of people be, to make it more likely than not that at least two of them share a birthday? The answer is 23.

The intuition behind this is quite straightforward if we recognise just how many pairs of people there are in a group of 23 people, any pair of which could share a birthday.

In a group of 23 people, there are in fact 253 pairs of people to choose from.  Therefore, a group of 23 people generates 253 chances, each of size 1/365, of having at least two people in the group sharing the same birthday.

To be technical about it, in a group of 23 people, there are, according to the standard formula, 23C2 pairs of people (called 23 Choose 2) pairs of people.

Generally, the number of ways k things can be chosen from n is:

n C k = n! / (n-k)! k!

Here n! (n factorial) is n x n-1 x n-2 … down to 1. Similarly for k!

Thus, 23C2 = 23! / 21! 2! = 23 x 22 / 2 = 253

These chances have some overlap: if A and B have a common birthday, and A and C have a common birthday, then inevitably so do B and C.

So the probability of at least two people sharing a birthday in a group of 23 is less than 253/365 (69.3%).

The probability that at least two people in the group of 23 do not share a birthday is:

(364/365)253 = 0.4995

Essentially, making 253 comparisons and having them all be different is like getting heads 253 times in a row, i.e. you avoided tails 253 times in a row.

The odds of two people having different birthdays is 1 – 1/365 = 364/365 = 0.99726.

The odds of 23 people having different birthdays is (364/365)253   = 0.4995

The odds that at least two of the 23 people share the same birthday = 1 – 0.4995 = 0.505 = 50.5%

So the next time you see two football teams line up, include the referee. It is now more likely than not that two of those on the pitch share the same birthday. Strange, but true!

Exercise

What is the probability that a randomly selected group of 24 people share a birthday? Assume that all dates in the calendar are equally likely as birthdays, and ignore the Leap Year February 29.

References and Links

Probability and the Birthday Paradox. Scientific American. March 29, 2012. https://www.scientificamerican.com/article/bring-science-home-probability-birthday-paradox/

Understanding the Birthday Paradox. Better Explained. https://betterexplained.com/articles/understanding-the-birthday-paradox/

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