A Critical Appraisal Dictionary
This concise dictionary collects core research measures, common study designs, sources of bias and key appraisal concepts encountered in exams and everyday ED practice. Each entry gives a short definition, the essential formula or rule where applicable, a single worked ED‑relevant example, and a brief practical note on how it affects interpretation.
Measures of effect and clinical impact
Absolute risk / Absolute risk reduction (ARR)
- Definition: Absolute risk = incidence (probability) of an outcome in a group. Absolute risk reduction (ARR) = difference in incidence between control and intervention groups (incidencecontrol - incidencetreatment).
- Formula: ARR = incidencecontrol - incidencetreatment.
- Example: In 100 major trauma patients, 30 placebo deaths vs 20 treatment deaths → ARR = 0.30 - 0.20 = 0.10 (10%).
- Practical note: ARR shows the real change in risk attributable to treatment and is the basis for the number needed to treat (NNT).
Relative risk (Risk ratio, RR) and Relative risk reduction (RRR)
- Definition: RR = incidence in treatment ÷ incidence in control. RRR = proportional reduction relative to control (1 - RR).
- Formula: RR = incidencetreatment / incidencecontrol. RRR = 1 - RR (or ARR ÷ incidencecontrol).
- Example: From above, RR = 0.20 / 0.30 = 0.67 → RRR = 33%.
- Practical note: RR/RRR can exaggerate perceived benefit when baseline risk is small-always report...
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