Primary Research: Investigators actively intervene with or observe what happens to the test subjects in a study design. In experimental trials such as randomized or non-randomized controlled trials, investigators use a prospective method of assigning an exposure, then compare treatment and control groups to perceive an outcome. Or, using observational studies, investigators observe what happens to test subjects, usually by retrieving information through a data registry to perceive relationships between exposures and outcomes. Cohort, Case-Control, and Cross-Sectional study designs are the most prominent of these.
Secondary Research: Review articles or other approaches that incorporate data from primary studies. Secondary studies gather information about what other researchers have done, then synthesize the information together for a more conclusive outcome. In specialized studies such as Systematic Reviews or Meta-Analysis, researchers answer a relevant clinical question using a reproducible methodology and evaluation of controlled study data. Further evaluation and integration of review articles can lead to tertiary evidence, such as with the synthesis of electronic records and systematic review summaries.
Syntheses: Filtered research in the form of summaries, or reviews that use explicit methods identify, integrate, and appraise studies for more conclusive analysis. Two examples are:
Randomized Controlled Trial: A study in which participants are randomly allocated into an experimental group or control group and followed over time for the variables/outcomes of interest.
Cohort Studies: Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. A cohort study will have a control group that represents the unexposed population.
Case Controlled Studies: A study which involves identifying patients who have the outcome of interest (cases) and patients without the same outcome (controls), and looking back to see if they had the exposure of interest.
Case Series/Reports: A report on a series of patients or single patient with an outcome of interest. No control group is involved.