Categories of evidence are conceptualized as a hierarchy, with the top levels considered the best evidence for clinical decision-making. These higher levels are considered secondary literature, which synthesizes, filters, and evaluates the primary (lower level) literature. The hierarchy can include:
- Systematic Reviews: A review of all research available on a given topic, conducted with strict methods of locating and synthesizing the research.
- Systematic Reviews with Meta-Analyses: Reviews that pool and summarize the data from each study included in the review.
- Critically-Appraised Topics: A critical appraisal of clinically relevant studies.
- Critically-Appraised Individual Articles: Articles that are selected and rated for clinical relevance by physicians.
- Randomized Controlled Trials: A study wherein the subjects are divided into two groups: one that received the genuine treatment, and one that receives a placebo treatment.
- Cohort Studies: A study that compares outcomes between two groups studied over a period of time.
- Case-Controlled Studies: A study that compares groups with differing outcomes with respect to their exposure to a suspected causal attribute.
- Case Series: A study that measures the outcome of a group of patients who receive the same treatment, with no control group.
- Case Reports: Reports on the treatment and outcome of a single patient.