[Math] The expected number of matches from a deck of cards

probabilityprobability distributionsstatistics

Suppose we have a normal deck of 52 cards. We shuffle them well and then turn over the first 13 cards one-by-one. If the first card is one of the four aces we say that a match has occurred; similarly, if the second card is one of the twos; the third card is one of the threes, etc.; until the 13th (one of the kings).

What is the expected number of matches?

my solution: (4/52) + (4/51) + … until the 13th draw.

Best Answer

One of the things that is difficult to grasp about expectations is that they are additive. Let $M_i$ be the number of matches in the position $i$ - so that $M_i$ is either $0$ or $1$.

Then $E(M_i)=\frac{4}{52}=\frac{1}{13}$.

Now, the total number of matches is $M=M_1+\cdots+M_{13}$. So, by additivity, $$E(M)=E(M_1+\cdots + M_{13}) = 13\cdot\frac{1}{13}=1$$

This additivity property is often confusing, because it seems to miss the conditional probabilities. It doesn't, but it takes some effort to understand why.