Poisson Null Models

The null hypothesis of a Poisson disease rate is usually a good representation of randomly distributed non-infectious rare diseases (Waller and Jacquez 1995). It is used in a few cluster detection methods, including Besag and Newell's method. A Poisson function can be described by one parameter, lambda ( l ), the mean and variance of the distribution. Lambda is the average or expected case count, calculated from the average or expected disease frequency multiplied by the population-at-risk.