The Number Needed to Treat (NNT) to prevent one adverse outcome is calculated as the reciprocal of:
- A Relative Risk Reduction (RRR)
- B Absolute Risk Reduction (ARR) ✓
- C Relative Risk (RR)
- D Odds Ratio (OR)
Correct answer: B. Absolute Risk Reduction (ARR)
Explanation
NNT = 1 / ARR, where ARR (Absolute Risk Reduction) = Risk in control group − Risk in treatment group. For example, if event rate in controls = 20% and in treated = 10%, ARR = 10% = 0.10, so NNT = 1/0.10 = 10. NNT based on RRR (RRR = ARR/control risk) would be: NNT = 1/(RRR × baseline risk) — which requires knowledge of baseline risk and is less direct. NNT is preferred over RRR for clinical decision-making because it incorporates baseline risk and gives an absolute sense of treatment benefit.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.