How this calculation works
Compare event rates across two groups that can have different exposure times.
THE FORMULA
RR=(x₁/t₁)/(x₂/t₂), CI=exp(ln RR ± z*√(1/x₁+1/x₂))A worked example
40 events versus 20 over equal exposure gives a rate ratio of 2.
Good to know
This large-sample interval requires positive counts and independent Poisson events.
Reference: OpenStax Introductory Statistics (opens in a new tab) ↗
Read the result
Compare event rates across two groups that can have different exposure times.
The primary result is labeled “Rate ratio”. Keep the input definitions in view when comparing this result with another method.
Try a related calculation
This large-sample interval requires positive counts and independent Poisson events.
For a related exercise, try the statistics calculator.
A few questions, answered
How can I check the default inputs by hand?
40 events versus 20 over equal exposure gives a rate ratio of 2.
Which rule does this calculator apply?
RR=(x₁/t₁)/(x₂/t₂), CI=exp(ln RR ± z*√(1/x₁+1/x₂))