Levels of Evidence is a ranking system used in evidence-based medicine to describe the strength of information. The higher the point in the pyramid the lower the the likelihood of bias in any statement/finding reported. Scroll to see the evidence types and to identify search tools that will help you locate resources to answer your questions.
Systematic review: A comprehensive survey of a topic whereby all primary studies of the highest level of evidence are systematically identified, appraised and then summarized according to an explicit and reproducible methodology.
Meta-analysis: The statistical technique involved in extracting and combining data to produce a summary result. The process combines the results from studies that are similar enough statistically then analyzes them as if they were from one study.
Critically-appraised topic: A brief summary of a search and critical appraisal of the literature related to a focused clinical question, used to help make clinical decisions.
Critically-appraised individual article: A systematic process used to identify the strengths and weaknesses of a research article in order to assess the usefulness and validity of research findings. Important components of a critical appraisal are an evaluation of the appropriateness of the study design for the research question and an assessment of the key methodological features of this design. Other factors considered include the suitability of the statistical methods and their subsequent interpretation, potential conflicts of interest and the relevance of the research to one's own practice.
Randomized controlled trial: A study in which participants are concurrently enrolled and randomly allocated into an experimental group or a control group to receive or not receive an experimental, preventive, or therapeutic procedure, maneuver, or intervention. Participants are followed over time for the variables/outcomes of interest. This type of study is the gold standard for testing the efficacy of an intervention.
Cohort study: Involves identification of two groups (cohorts) of patients, one which received the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.
Case control study: Involves an observational study of persons who have the outcome of interest (cases) and persons without the same outcome (controls), and looking back to see if they had the exposure of interest.
Case series / Case report: A report on a series of patients or an individual patient with an outcome of interest. No control group is involved. Reports of case series usually contain detailed demographic information and information on diagnosis, treatment, response to treatment, and follow-up after treatment.
Observational study: A family of studies comparing people who take an intervention with those who do not. Investigators neither allocate persons to receive the intervention nor administer the intervention. Instead, they compare records of persons who had taken an intervention with similar patients who had not. The most common observational designs are case-studies, case-series, case-control studies, cohort studies, and historically controlled studies. (Howick)