Part One: Probability

Conditional Probability

How to compute a conditional probability

The basic formula,

Pr{ A | B }=Pr{ A and B } / Pr{ B }.

Suggests the following steps:

I Identify the prior event, B. Then compute Pr{ B }

II Identify the event A, and the joint event A and B. Then compute Pr{ A and B }.

III Finally, compute the ratio Pr{ A | B } = Pr{ A and B } / Pr{ B }.

In certain special cases, we can compute Pr{ A | B } directly.