TU, Germany

The terms “effectiveness” and “efficacy” seem to be used as synonyms and to be quite interchangeable, despite formal conceptual differences between both. To my knowledge this might be due to translation difficulties (in German for example “Efficacy” is usually translated as “Wirksamkeit” and “Effectiveness” as “Wirksamkeit unter Alltagsbedingungen”, which is then too often shortened leaving “Wirksamkeit” alone again). The confusion might be also due to lack of clarity on interpreting whether the conditions of a trial were so far a way from conditions in every day practice. Thus in front of a piece of evidence it might be difficult to separate both terms clearly, this leading to interchangeable use of both.

Some separate both concepts depending on how the evidence was gathered; speaking of “Efficacy” when they refer to the effect measured in a randomized controlled trial and of “Effectiveness” when the effect has been measured based for example on routinely collected data (epidemiological, administrative, etc.). Another way to separate both concepts might be along the phases of drug developing i.e. depending of the results come from a Phase II, III or IV trial, this however could only be applied to drugs since the phase differentiation does not apply to other kinds of interventions.

Other differentiations can be made on the basis of whether the data analysed refer only to the persons who got the intervention (analysis per protocol) or in contrast the persons who did not get the intervention (intention-to-treat analysis). The former would give an estimate of “Efficacy” the latter would be closer to the concept of “Effectiveness”.
In order to facilitate adaptation following simplified definitions of both concepts could be taken as a starting point:

- “Efficacy” effects of an intervention as measured in a situation unlikely to be widely found.

- “Effectiveness” effects of an intervention as measured in a situation similar or very similar to common practice.

Following this understanding, a report should speak about “Efficacy” when referring to evidence of effects gathered in studies whose conditions are very artificial and not likely to be found in common practice (independently of whether they were RCTs or other kinds of studies).

“Effectiveness” should be used when referring to evidence of effects gathered in studies in which the conditions are similar or very similar to common practice (independently of whether they were RCTs or other kinds of studies). One can also speak about “Efficacy” when the assessment has modeled the effect taking evidence from studies on “Efficacy” as starting point and adding evidence from other sources on other terms of the equation (e.g. compliance, diagnostic accuracy, etc.).