Efficacy is the capacity for beneficial change (or therapeutic effect) of a given intervention, most commonly used in the practice of medicine and pharmacology.
Efficacy of a technique or product arises from the results obtained by a given center or professional who is outstanding on this subject.
Usually, the randomized trial is the appropriate study to verify efficacy because it provides greater control of the possible confounding variables. Effectiveness is the capacity to reproduce and obtain the same results within the medical community, using different centers and with professionals who have distinct degrees of experience. The observational study is generally used, because it selects patients with more heterogeneous characteristics and centers with different expertise.