Introductory Data Science using R

R Exercise: The birthday problem

In a room of 23 people, what is the probability that at least two people share the same birthday?

Let’s count

First, some assumptions:

  • There are only 365 days in a year
  • Every day is equally likely to be a birthday
  • Everyone’s birthday is independent of each other
Strategy: It’s easier to figure out the probability of the complementary event. $$P(A) = 1 - P(A^c)$$

What’s the complement?

  • Let $A$ = At least two people share the same birthday
  • Then $A^c$ = Nobody shares any birthday (all birthdays are different)
  • Label the individuals from $1,\dots,23$
  • How many possible birthdays can person 1 have? 365 out of 365
  • How many possible birthdays can person 2 have? 364 out of 365
  • …

What’s the complement?

  • Since all events are independent, $$P(A^c) = \frac{365}{365} \times \frac{364}{365} \times \cdots \times \frac{365-23+1}{365}$$ $$= \frac{365!}{(365-23)!365^{23}}$$
  • Thus, $$P(A) = 1 - \frac{365!}{(365-23)!365^{23}}$$

Logarithms

Factorials are often too large to compute and can cause memory overflow. Adopt the alternative formula

$$P(A) = 1 - \exp \big\{ \log(365!) - \log((365-23)!) $$ $$- 23 \log 365 \big\}$$

Write this in R

Functions that you need:

  • factorial() to compute factorials
  • lfactorial() to compute log factorials
  • exp() to compute exponentials

New question

In a room of $x$ people, what is the probability that at least two people share the same birthday?

Write this in R

Write a function that takes a positive integer x and returns the probability that at least two people share the same birthday.

BONUS: Plot it!