Evidence-based dental practice: Part II. Levels and quality of evidence, and research designs.

The ability to make a sound clinical decision is based largely on the quality of evidence and the practitioner's ability to evaluate this evidence. Clinical evidence are categorized and ranked according to the strength of their freedom from the various biases that beset medical research. Randomized controlled trials (RCTs) are the "gold standard" by which all clinical research is judged. Systematic reviews/meta-analysis of randomized controlled trials where available are considered the highest level in the evidence hierarchy (clinical questions related to therapy or interventions). However, this hierarchy of evidence can not answer all clinical questions especially the ones regarding diagnosis, aetiology or prognosis. For questions related to diagnosis, prognosis or causation, other study designs such as longitudinal studies, cohort studies or case-control studies are more appropriate. The present article discusses the levels and quality of evidence, and basic concepts of clinical research design in evidence-based dental practice based on review of existing literature.