Measures of effect - absolute and relative risk

Definitions

Absolute risk is the actual probability of an outcome occurring in a specific group over a defined period; relative risk is the ratio of risk in an exposed/treated group compared with an unexposed/control group - relative risk describes proportional change, while absolute risk describes the real-world magnitude of effect, and both are needed together for a complete, clinically meaningful picture
Among the most frequently reported and, unfortunately, frequently misinterpreted effect measures in clinical research and health reporting/media coverage

A treatment can produce an impressive-sounding relative risk reduction (e.g. "50% reduction in risk") while the underlying absolute risk reduction is clinically trivial if the baseline absolute risk was already very low (e.g. reducing risk from 0.2% to 0.1% is a 50% relative reduction but only a 0.1 percentage point absolute reduction) - this is why relative risk figures alone, without the accompanying absolute risk context, can create a misleading impression of clinical benefit magnitude

Both measures should be reported together for complete interpretation - relative risk for understanding the strength/consistency of an association or treatment effect, absolute risk (and number needed to treat, derived from absolute risk difference) for understanding real-world clinical impact magnitude
Media and promotional reporting of clinical trial results frequently emphasises relative risk reduction alone, given its more impressive-sounding numbers, requiring critical appraisal to identify the accompanying absolute risk context

Treatment decisions should weigh absolute risk reduction (and the resulting number needed to treat) alongside relative risk reduction, since absolute measures better reflect real-world clinical impact for an individual patient's actual baseline risk
A patient's individual absolute risk (which may differ from the trial population's average) should inform how much absolute benefit they personally can expect from a treatment with a given relative risk reduction

Media reporting emphasis on relative risk reduction (a common source of misleading health communication), absolute risk reduction and number needed to treat derivation, baseline risk's effect on absolute benefit magnitude for a given relative risk reduction

The same relative risk reduction translates into very different absolute benefit depending on baseline risk - a 50% relative risk reduction is clinically meaningful when baseline risk is high (e.g. 20% reduced to 10%, a substantial absolute benefit) but nearly negligible when baseline risk is very low (e.g. 0.2% reduced to 0.1%), which is why relative risk figures reported in isolation, without baseline/absolute risk context, can be genuinely misleading about real-world clinical impact

5 more sections, plus exam facts

Premium unlocks every note across every specialty, and the full exam fact library behind it.