Search for current systematic reviews, clinical guidelines, critical summaries, and meta-analyses from respected sources first. These documents synthesize results from multiple individual studies to offer the best founded evidence for practice.
Hierarchy of evidence for clinical questions related to therapy, prevention, etiology or harm
Always start an EBD search looking for the highest level of evidence that matches your query needs. If a meta-analysis is not available on the topic, look next for systematic reviews without statistical synthesis, next for randomized control trials, next for cohort studies, next for case control studies, etc.
For more detailed information about the levels of evidence see the Oxford Centre for Evidence-based Medicine - Levels of Evidence (March 2009). Drawn from the UNC Chapel Hill Evidence-Based Dentistry Guide.