If a model’s outcome is a count of events, the window of opportunity for events to occur may vary for each record. In these cases, you can select an "exposure variable" that reflects each record’s relative exposure to the event process. Typically, the exposure variable will be an area, time span, or population count.
To set the exposure variable:
Note that values for the exposure variable must be greater than zero. Records with an exposure of zero (or a negative value) will be excluded from the model estimation.