Is All Evidence Created Equal? No.
The medical literature is immense, but only a small portion of it is immediately useful in answering clinical questions. The literature reports the whole spectrum of the scientific research process -- the long journey from in-vitro studies to double-blind randomized controlled trials. This has been called the "wedge of evidence" or the "pyramid of evidence." (See below) Also see the Oxford Centre for Evidence-based Medicine for another chart.
An understanding of how various levels of evidence are reported and how this literature is organized will help the searcher retrieve the highest levels of evidence for a particular clinical question. High levels of evidence may not exist for all clinical questions because of the nature of medical problems and research and ethical limitations.
Adding METHODOLOGY terms and CLINICAL FILTERS to SUBJECT terms will result in the most efficient and optimal retrieval in terms of finding the highest level of evidence in answering clinical questions.