Absolute Risk Reduction (ARR) and NNT Calculator
Calculate ARR, RRR, NNT, and Relative Risk from control and treatment event rates for interpreting clinical trial results and evidence-based medicine.
Understanding Risk Reduction Metrics
When evaluating a treatment or intervention, doctors and researchers use several measures to describe how much benefit is provided. Each tells a different part of the story.
The Four Key Measures
Control Event Rate (CER): the event rate in the control (untreated) group. Experimental Event Rate (EER): the event rate in the treated group.
Absolute Risk Reduction (ARR) ARR = CER − EER
This is the most honest measure — it tells you the actual percentage point difference.
Relative Risk Reduction (RRR) RRR = ARR ÷ CER × 100%
This compares the reduction relative to the baseline risk. Drug advertising often highlights RRR because it sounds impressive.
Relative Risk (RR) RR = EER ÷ CER
If RR < 1, treatment reduces the event rate.
Number Needed to Treat (NNT) NNT = 1 ÷ ARR (when using decimals, not percentages)
NNT tells you how many patients must be treated to prevent one additional event.
Worked Example
A cholesterol drug trial: CER = 10%, EER = 6%
- ARR = 10% − 6% = 4%
- RRR = 4% ÷ 10% = 40% (sounds great in ads!)
- RR = 6% ÷ 10% = 0.60
- NNT = 1 ÷ 0.04 = 25 (25 people treated to prevent 1 event)
NNT Interpretation Guide
| NNT | Interpretation |
|---|---|
| 1–5 | Highly effective, nearly every patient benefits |
| 5–15 | Very effective, major clinical impact |
| 15–50 | Moderately effective, worthwhile treatment |
| 50–100 | Modest benefit, depends on severity of disease |
| Over 100 | Low benefit, question whether treatment is warranted |
The NNT should always be evaluated alongside the severity and cost of the disease and treatment.