how well a treatment, intervention, or program works in real-world conditions among a representative, heterogeneous population. Unlike efficacy—which measures outcomes under ideal or controlled circumstances—effectiveness captures the success of a treatment when used in typical practice settings with patients who have varying characteristics.
Example
A new diabetes medication shows strong results in tightly controlled clinical trials (high efficacy). When the same drug is prescribed to a broad population with diverse backgrounds and coexisting conditions, researchers conduct an effectiveness study to see if it produces similar benefits in everyday healthcare settings.