Estimate absolute risk reduction and number needed to treat from two event rates.
| ARR (points) | – |
| NNT | – |
Use this calculator for ARR = control event % − treatment event %, then NNT = 100 ÷ |ARR|. Event rates are percentages (0–100), not fractions. If the two rates are equal, NNT is undefined and the calculator errors. A negative ARR (treatment worse) still yields a positive NNT of harm. Type the two rates; do not paste raw counts. This is not a vaccine-efficacy (1 − RR) page.
Control event (%): Event rate in the control arm, percent.
Treatment event (%): Event rate in the treatment arm, percent.
ARR: Absolute risk reduction in percentage points.
NNT: Number needed to treat (or harm) for one extra event.
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