276 Statistics and Data Analysis for Microarrays Using R and Bioconductor
2. The occurrence of an event in any period of time (or region of space) is in-
depe ndent of the occur rence of the event in any other (no n-overlapping)
period of time (or re gion of space).
3. The expe cted numbe r of events during any one period of time (o r region
of space) is constant. This expected number of event occurrences in the
given interval or region is a parameter of the Poisson distribution and is
denoted by λ.
A process that satisfies conditions 1 and 2 above is also known as a Poisson
process. If condition 3 is als o true, the process is a homogeneous Poisson
process.
The clas sical example of a phenomenon that is well modeled by a Poisson
distribution is the