Comprehensive Calculator Guide
This calculator compares the risk of an outcome between exposed (or treated) and unexposed (or control) groups: relative risk, absolute difference, relative change and number needed to treat. 15/1,000 vs 30/1,000 gives RR 0.5 but only a 1.5-point absolute drop: NNT 67.
Relative vs absolute
RR divides the risks; the absolute difference subtracts them; NNT = 1 ÷ |difference|. Headlines quote relative changes; always ask for the absolute ones. A 95% CI excluding 1 suggests a real difference.
How to Use
- Enter cases and size of the exposed group.
- Enter cases and size of the unexposed group.
- Press “Calculate” to see RR, the absolute difference and NNT.
Formula Used
RR = (a/n1) ÷ (c/n2) | ARD = a/n1 − c/n2 | NNT = 1/|ARD|
Practical Examples
- Example 1
- 15/1,000 vs 30/1,000: RR 0.5, NNT 67.
- Example 2
- 60/500 vs 20/1,000: RR 6.
Important Tips
- Report absolute risk too.
- Compare similar groups.
- Use large samples.
Common Mistakes to Avoid
- Confusing RR with odds ratios.
- Quoting only relative change.
- Inferring cause from observation.
Frequently Asked Questions
- RR of 2?
- Twice as common.
- What is NNT?
- Treated per case prevented.
- RR vs OR?
- Close when the outcome is rare.