One empty event compartment beside another cohort sample tray. Original AI-generated artwork produced for GreenCalc; not a measured diagram or empirical data display.
Separate an undefined ratio from an infinite one
If both groups have zero observed events, both observed risks are zero and their ratio is 0/0, which is undefined. If group 1 has a positive observed risk and the reference group has zero, the observed RR is infinite. If only group 1 has zero risk and the reference risk is positive, observed RR is zero. These boundary results are not interchangeable.
Why the ordinary log interval stops
The approximate log RR standard error contains 1/a and 1/c, where a and c are event counts. A zero event cell makes that expression unavailable, and log zero is also not finite. The Relative Risk Calculator therefore omits this interval when either event cell is zero rather than silently adding a correction to the data.
Do not treat no observed events as impossible events
A finite sample can contain zero events even when the population event probability is positive. An infinite sample ratio also does not establish an infinitely large causal effect. Small counts, selection, follow-up and uncertainty need appropriate consideration. Alternative interval methods can be considered in a proper analysis, but their assumptions and any continuity correction must be disclosed.
Keep the report interpretable
Show both groups' event counts and denominators alongside the ratio and describe the interval method. Read reference direction and follow-up for the estimand definition. The P-Hat Calculator provides the related sample-proportion interpretation, including why a zero plug-in standard error does not establish population certainty.
Check the model behind your result
Use the Relative Risk Calculator for the supported calculation and review its stated assumptions before interpreting the output. Find related tools in the Statistics category, or browse the GreenCalc homepage for other calculation topics.
By GreenCalc Editorial Team · Updated 2026-10-26