Independent Poisson Rate Ratio
Compare event counts per exposure with an approximate log-Wald 95% interval.
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Compare event counts per exposure with an approximate log-Wald 95% interval. All default values are fictional.
Calculation results
Rates per exposure unit: 0.03 and 0.02.
Rate ratio: 1.5; approximate 95% interval: 0.851865 to 2.64127.
Assumptions and limits
Rate ratio = (events1/exposure1)/(events0/exposure0). The 95% log-Wald interval is exp(log(ratio) ± 1.959964 sqrt(1/events1+1/events0)). Requires independent Poisson counts, comparable exposure units, and fixed known exposure. This large-count approximation can be poor for sparse counts. Zero counts are rejected rather than silently corrected; use a suitable exact or score method. Not adjusted for confounding, overdispersion or dependence among recurrent events. Rates are not risks.
statsmodels Poisson rate intervals · Read the practical guide · Download blank worksheet (TXT)