Absolute Risk Reduction (ARR) and NNT Calculator
Calculate Absolute Risk Reduction (ARR), Relative Risk Reduction (RRR), Number Needed to Treat (NNT), and Relative Risk (RR) from control and treatment event rates.
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.