Definition
Cost-effectiveness is the practice of comparing interventions by their outcome per dollar spent — lives saved per million, learning-years gained per thousand, emissions cut per thousand — rather than by their absolute outcome.
It is Rosling's corrective for the Size Instinct in policy: a small intervention that delivers a large effect per dollar dominates a big intervention that delivers a small effect per dollar, even when the latter sounds bigger in headlines.
Why it matters
How it works
The two operative numbers are cost and outcome. Costs are reasonably comparable across interventions (dollars, person-hours), so the work is mostly in defining outcome in a way that travels: lives saved, disability-adjusted life-years (DALYs), tonnes of CO2 averted, schooling-years gained. Once outcome is in a common currency, dividing it by cost ranks the field.
Discount rates and time horizons matter: a treatment that delivers benefit over 50 years versus one that delivers benefit in a single year are not directly comparable in raw dollars per outcome. DALYs and similar measures attempt to fold time into the outcome metric.
The chief failure mode is mistaking the most legible cost-effectiveness analysis for the truth. Famously cheap interventions (vaccines, bed nets, salt iodisation) are well-measured because they are simple; complex structural interventions are harder to score and therefore frequently miss the league table even when their effects may be larger. Use cost-effectiveness as a sort key, not a final verdict.