Working Procedures


2. Scope

Cochrane (1972) first discussed the concepts of efficacy and effectiveness in the context of health interventions. “Efficacy” was later defined by Porta (2008) as "the extent to which a specific intervention, procedure, regimen, or service produces a beneficial result under ideal conditions; the benefit or utility to the individual or the population of the service, treatment regimen, or intervention. Ideally, the determination of efficacy is based on the results of a randomized controlled trial". In contrast, the related term "effectiveness" is defined by the same author as "a measure of the extent to which a specific intervention, procedure, regimen, or service, when deployed in the field in the usual circumstances, does what it is intended to do for a specified population; a measure of the extent to which a health care intervention fulfils its objectives in practice". The distinction between efficacy as measured in experimental studies and the effectiveness of a mass population intervention is a crucial one for public health decision-making. In particular, the fact that the effectiveness of a screening procedure may be different in different populations is often overlooked. A mass programme of screening must satisfy certain minimal requirements (e.g. acceptability, availability of relevant personnel, facilities for screening, and access to pertinent health services) if it is to achieve the results that have been documented in epidemiological studies.

The acceptance and use of screening services may vary from one population to another, implying that a given screening procedure is not universally effective. Even when a screening procedure is effective as a mass intervention, other outcomes, such as harm and costs and the potential for other interventions to achieve equivalent benefits, must be considered. Efficacy is a necessary but not sufficient basis for recommending screening. The efficacy of a screening procedure can be inferred if effectiveness can be proven. Screening has sometimes been implemented by a given procedure on the assumption that “earlier is better”, even when no evidence of efficacy was available. If such interventions result in a significant reduction in mortality that cannot otherwise be explained, it can be inferred that the procedure is effective. However, uncontrolled interventions in which individuals are exposed to unknown risks and benefits should be avoided.


Updated 10 October 2014