This past Monday, we introduced conditional probability in our Blackboard Collaborate session. Here is a brief recap, with some online resources included below that go over the definition and go through some examples.

The conditional probability of A *given* B is defined as:

*P(A|B) = P(A & B)/P(B)*

Note that by switching A and B, we can also look at the conditional probability of B given A:

*P(B|A) = P(A & B)/P(A)*

(Note that the numerator is the same in both cases, since *P(A&B) = P(B&A)*. The denominator is the probability of the “condition” i.e., the event after the vertical line “|”.)

The following 2 videos may be helpful:

If you have a copy of the textbook (Ross), you should read the examples listed on the class outline pdf. I will try to update this post with some additional online examples shortly.