When a meteorologist provides a weather forecast, precipitation is typically described with terms such as "70 percent chance of rain." Such forecasts are known as probability of precipitation reports. Have you ever considered how they are calculated? It is a puzzling question, because in reality, either it will rain or not.

Weather estimates are based on probabilistic methods or those concerned with describing uncertainty. They use data on past events to extrapolate future events. In the case of weather, the chance of rain describes the proportion of prior days to similar measurable atmospheric conditions in which precipitation occurred. A 70 percent chance of rain implies that in 7 out of the 10 past cases with similar conditions, precipitation occurred somewhere in the area.

This chapter covers the Naive Bayes algorithm, which uses probabilities in much the same way as a weather forecast. While studying this method, you...