Calculator inputs
Enter a 2×2 cohort table. Results appear above this form after submission.
Example data table
This example shows a typical cohort-style input structure.
| Group | Events | Non-events | Total | Risk |
|---|---|---|---|---|
| Exposed | 42 | 158 | 200 | 21.00% |
| Unexposed | 18 | 182 | 200 | 9.00% |
| Incidence risk ratio | 2.3333 | |||
Formula used
Riskexposed = a / (a + b)
Riskunexposed = c / (c + d)
RR = Riskexposed / Riskunexposed
RD = Riskexposed − Riskunexposed
SE[ln(RR)] = √[(1/a) − 1/(a+b) + (1/c) − 1/(c+d)]
CI = exp( ln(RR) ± z × SE[ln(RR)] )
How to use this calculator
- Enter event and non-event counts for the exposed group.
- Enter event and non-event counts for the unexposed group.
- Select your preferred confidence level.
- Enable continuity correction if you expect zero cells.
- Click the calculation button.
- Review the ratio, interval, risk difference, and interpretation above the form.
- Use the CSV or PDF buttons to export the results.
- Inspect the graph to compare group risks visually.
Notes for interpretation
When the ratio is above 1
A value above 1 means the exposed group has a higher cumulative incidence than the unexposed group.
When the interval includes 1
If the confidence interval crosses 1, the observed difference may not be statistically clear at the chosen confidence level.
Frequently asked questions
1) What does incidence risk ratio measure?
It compares cumulative incidence between two groups. The exposed group risk is divided by the unexposed group risk to show relative increase or decrease.
2) When should I use this calculator?
Use it for cohort-style data when participants are grouped by exposure and followed for whether an event occurred during a defined period.
3) What is the difference between risk ratio and odds ratio?
Risk ratio compares actual cumulative risks. Odds ratio compares odds, which can exaggerate effect size when events are common.
4) Why is there a continuity correction option?
Zero cells can make logarithmic calculations unstable. The continuity correction adds a small value to each cell when needed.
5) What does a ratio below 1 mean?
It means the exposed group has lower cumulative incidence than the unexposed group, suggesting a possible protective association.
6) Why does the calculator report risk difference too?
Risk difference shows the absolute change in cumulative incidence. It complements the ratio by revealing the size of the practical effect.
7) Can I use decimal values instead of whole numbers?
Yes. The form accepts numeric values, although whole participant counts are most common and easiest to interpret in epidemiologic tables.
8) What does it mean if the confidence interval includes 1?
It means the observed relative difference may be compatible with no association at the selected confidence level.