Compare two observed group risks with a risk ratio, risk difference and supported approximate 95% log interval.
How to use this tool
- Choose the event or model and review the assumptions below.
- Enter the labeled inputs.
- Select Calculate and read the labeled outputs.
- Change an input and calculate again; Reset restores the example inputs.
Calculation method
RR=[a/(a+b)]/[c/(c+d)]. The approximate 95% log interval uses SE(log RR)=√[1/a−1/(a+b)+1/c−1/(c+d)].
Assumptions and limits
A comparison of risks over comparable follow-up in two groups, not an odds ratio or causal effect by itself. The log interval is an approximation and is omitted for zero event cells. No automatic continuity correction is applied.
Worked example
Counts 20 events and 80 non-events versus 10 events and 90 non-events give risks 20% and 10%, RR 2, and a risk difference of 10 percentage points.
Worked reading and practical questions
- Relative Risk Depends on the Reference Group and Follow-Up
- Zero Event Cells and the Limits of a Log Risk-Ratio Interval
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Frequently asked questions
What does this calculator assume?
A comparison of risks over comparable follow-up in two groups, not an odds ratio or causal effect by itself. The log interval is an approximation and is omitted for zero event cells. No automatic continuity correction is applied.
How do I enter and update the inputs?
Use the labeled fields, then select Calculate. Percent fields use 0 to 100. After editing, calculate again; Reset restores the example inputs.