Effectiveness versus efficacy: more than a debate over language.

As the physical therapy profession continues the paradigm shift toward evidencebased practice, it becomes increasingly important for therapists to base clinical decisions on the best available evidence. Defining the best available evidence, however, may not be as straightforward as we assume, and will inevitably depend in part upon the perspective and values of the individual making the judgment. To some, the best evidence may be viewed as research that minimizes bias to the greatest extent possible, while others may prioritize research that is deemed most pertinent to clinical practice. The evidence most highly valued and ultimately judged to be the best may differ based on which perspective predominates. One issue that highlights the importance of perspective in judging the evidence is the difference between efficacy and effectiveness approaches to research. These terms are frequently assumed to be synonyms and are often used incorrectly in the literature. There is actually a meaningful distinction between efficacy and effectiveness approaches to research. The distinction is not merely a pedantic concern within the lexicon of researchers, but impacts the nature of the results disseminated by a study, how the results may be applied to clinical practice, and finally how the results are judged by those who seek to evaluate the evidence. Understanding the contrast between effectiveness and efficacy has important and very practical implications for those who seek to evaluate and apply research evidence to clinical practice.