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Tuesday, February 11, 2014 CC-BY-NC
Discrete probability

Maintainer: admin

Introduction to discrete probability. The first part of the class was a quiz on discrete probability.

Notes originally by @xiamx, with some formatting/language changes by @dellsystem.

1Probability

The study of probability was initially motivated by gambling. We will consider probability w.r.t. finite probability spaces (or finitely countable spaces). A sample space S is the set of all possible outcomes of an experiment. For example, for a die, the sample space is S={1,2,3,4,5,6}.

An event A is a subset of the sample space S.

We associate with each outcome xS the probability p(x) that x is the outcome of the experiment. We say that p:2SR is a valid probability distribution on S if:

  1. p(x)0 xS
  2. xSp(x)=1
  3. p(A)=xAp(x)

Sometimes, the outcomes are all equally likely, with p(x)=1|S|xS (for example, p(x)=16 for any number x on a die). Usually, though, this is not the case.

1.1Examples

What is the probability of winning Lotto 6/49, in which the goal is to correctly pick 6 numbers, all in {1,,49}?
Well, the size of the sample space is \binom{49}{6} = 13983816. So the probability is \frac{1}{13983816}.
What is the probability of a full house in a game of poker?
The sample space is \binom{52}{5} = 2598960. The number of ways to get a full house is 13 \cdot \binom{4}{3} \cdot 12 \cdot \binom{4}{2} = 3744, so the probability is \frac{3744}{2598960}.

1.2Union and intersection

Lemma: For any probability distribution P:S \to \mathbb{R} and any 2 events A and B, we have:

p(A\cup B) = p(A) +p(B) - p(A \cap B)

The proof is trivial using set algebra.

Furthermore:

P(A \cup B) = P(A-B) + P(B-A) + P(A\cup B)