Many studies are based on counts of the number of nonconformities or defects per area of opportunity. An area of opportunity is a continuous unit of time, volume, or other such area in which more than one occurrence of an event can occur. Examples are the surface defects on a new refrigerator, number of complaints in a hotel in a week, or number of accidents in a factory in a month. In such circumstances, you can use the Poisson probability distribution to calculate probabilities. This includes applications to the theory of area of opportunity control charts that will be discussed in Section 11.6. Exhibit 5.2 describes the properties that must be true about a specific situation to use the Poisson distribution.