NNT, NNH & ARR - essential concepts for practice and exams
Absolute measures translate trial results into clinically useful terms. Absolute Risk Reduction (ARR), Number Needed to Treat (NNT) and Number Needed to Harm (NNH) tell you how many patients will benefit or be harmed by an intervention over a specified time period.
They complement relative measures (for example, relative risk reduction, RRR) and are preferred for shared decision‑making and clear communication (NICE, RCEM).
Use these measures to:
- convert trial results into patient‑level impact.
- compare benefits and harms side‑by‑side.
- communicate outcomes clearly with patients using natural frequencies.
Definitions and core formulae
- Control Event Rate (CER) = events in control group / control group size (proportion).
- Experimental Event Rate (EER) = events in intervention group / intervention group size (proportion).
- Absolute Risk Reduction (ARR) = CER - EER.
- Absolute Risk Increase (ARI) = EER - CER (use when the intervention increases risk).
- Number Needed to Treat (NNT) = 1 / ARR (ARR expressed as a proportion).
- Number Needed to Harm (NNH) = 1 / ARI (ARI expressed as a proportion).
Notes on units and conventions
- Convert percentages to proportions before calculations (for example, 0.83% = 0.0083).