Odds Ratio and Relative Risk

Introduction

• A pharmacology thesis, pharmacovigilance signal, or drug-safety/effectiveness study almost always ends in a single number that quantifies how strongly an exposure (drug, risk factor, treatment) is associated with an outcome (disease, adverse event, response) — the measure of association.
• Study design dictates the measure — a cohort study or clinical trial yields incidence, so it can produce a risk ratio (relative risk), risk difference, rate ratio or hazard ratio; a case-control study cannot estimate incidence and is limited to the odds ratio; a cross-sectional study measures only prevalence → the prevalence ratio.
• Common skeleton — all three frequency measures — risk, odds, rate — share the same numerator (number developing the dichotomous outcome) and differ only in the denominator; understanding the denominator is the key to choosing and interpreting the association measure.