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. 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).
"Meta-analysis is a tool by which small, related studies are identified and, through systematic review, are combined into a common data pool for a single, larger population analysis."
Livingston M,et al.Meta-analysis: an introduction into a research process. Special Care in Dentistry 28, no. 4 (2008):125-30. UNC-CH Full Text
"Systematic reviews summarize and synthesize the available evidence related to diagnosis, therapy, prognosis, and harm for clinicians, patients, and decision makers. Such reviews represent one of the most powerful tools to translate knowledge into action."
Carrasco-Labra A, Brignardello-Petersen R, Glick M, Guyatt GH, Azarpazhooh A. A practical approach to evidence-based dentistry: VI: How to use a systematic review. Journal of the American Dental Association. 2015 Apr;146(4):255-65.e1. UNC-CH Access
Randomized Controlled Trial (RCT)
"At the level of primary studies, RCTs represent the optimal study design to address questions of therapy. An RCT is an experiment assessing a medical treatment in patients. In an RCT, participants are allocated randomly into 2 or more groups that are treated equally except for the intervention the participants receive"
Brignardello-Petersen R, Carrasco-Labra A, Glick M, Guyatt GH, Azarpazhooh A. A practical approach to evidence-based dentistry: III: how to appraise and use an article about therapy. Journal of the American Dental Association. 2015 Jan;146(1):42-49.e1. UNC-CH Full Text
"A cohort study is one in which a group of subjects, selected to represent the population of interest, is studied over time."
Levin, K. A. Study design IV: Cohort studies. Evidence-based dentistry 7, no. 2 (2006):51-52. UNC-CH Full Text
"Like cohort studies, the purpose of case-control studies is to establish association between exposure to risk factors and disease. Unlike cohort studies, however, members of the population with the disease are selected into the study at the outset and risk factor information is collected retrospectively"
Levin, K. A. Study design V: Case-control studies. Evidence-based dentistry 7, no. 3 (2006):83-84. UNC-CH Full Text