The reasoning: relative vs absolute
| Measure | Formula | Answers |
|---|---|---|
| Relative risk (risk ratio) | Iₑ / Iᵤ | Strength of association; aetiology |
| Rate ratio | Rateₑ / Rateᵤ | Same, with person‑time |
| Odds ratio | ad / bc | Case‑control analogue; ≈ RR if outcome rare |
| Attributable risk (risk difference) | Iₑ − Iᵤ | Excess risk in exposed, absolute |
| Attributable fraction (exposed), or AR% | (Iₑ − Iᵤ)/Iₑ = (RR − 1)/RR | Share of exposed cases due to exposure |
| Population attributable risk | I_total − Iᵤ | Excess incidence in the population |
| Population attributable fraction (PAF) | (I_total − Iᵤ)/I_total; Levin: Pe(RR − 1) / [1 + Pe(RR − 1)] | Share of all cases preventable by removing exposure |
| NNT / NNH | 1 / absolute risk reduction (or increase) | People treated (exposed) per one outcome prevented (caused) |