Table 1.

Expected variation due to chance predicted from the Poisson distribution for different counts of events.

Counts	Size of variation covering 95% of units
8 or less	>fourfold
9–14	threefold to fourfold
15–33	twofold to threefold
34–90	1.5-fold to 2-fold
91–214	1.3-fold to 1.5-fold
215–462	1.2-fold to 1.3-fold