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# Conditional Probability

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*Probability* is a way of
measuring how likely something is to occur. It is usually written as
*Pr(A) = x*, where A is an event. For example, we
might say that there’s a 20 percent chance of rain today, which would be
written as *Pr(rain) = 0.2*.

If we were to note that it’s already cloudy right now, then we
might conclude there’s a higher chance of rain later today. This is
called *conditional probability*,
which is the chance of A given that we know B. This is written as
*Pr(A | B)*, so in this case, it’s *Pr(rain
| cloudy)*.

The formula for conditional probability is the probability of both events happening divided by the chance of the given condition, as shown in Figure B-5.