- noun how well something works
- noun the quality of working successfully or producing results
- (written as Effectiveness)A measure similar to efficacy except that it refers to the effect of a particular medical technology or procedure on outcomes when used in 'actual' practice. It thus differs from efficacy in that efficacy concerns only the technical relationship between the procedure and its effects under 'ideal' conditions (in practice, typically the conditions that obtain in a research-oriented teaching hospital or primary care practice). 'Actual' practice is thus conceived to be practice as conducted by average professionals working with average resources. In economic jargon, the idea of effectiveness is encompassed within the notion of a production function. In epidemiology, clinical trials are typically designed to generate high levels of internal validity (efficacy) and pragmatic trials high levels of external validity or generalizability.