Difference between efficacy study and effectiveness studies

It is worth mentioning here the difference between efficacy studies and effectiveness studies. They are both important when evaluating interventions but they serve different purposes and have different study designs. All too often people get these two confused.

Efficacy studies investigate the benefits and harms of an intervention under highly controlled conditions. These studies create high internal validity, they often however require substantial deviations from clinical practice, including restrictions on the patient sample, control of the provider skill set and limitations on provider's action. The intervention is often compared to placebo.

Effectiveness studies (also known as pragmatic studies) examine interventions under circumstances that are closer to what happens in practice, with more heterogeneous populations, less-standardised treatment protocols and they take place in routine clinical settings. The intervention is most often compared to usual care.

The efficacy study reveals efficacy, the magnitude of the effect may well be different in practice, and depending on who is being treated (i.e. which patients).