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 |