A Meta-analysis will thoroughly examine a number of valid studies on a topic and combine the results using accepted statistical methodology to report the results as if it were one large study. The Cochrane Collaboration has done a lot of work in the areas of systematic reviews and meta-analysis.
Systematic Reviews usually focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify all potentially relevant studies. The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question. Click here to learn more about conducting systematic reviews.
Randomized Clinical Trials (RCTs) are carefully planned projects that introduce a treatment or exposure to study its effect on real patients. They include methodologies that reduce the potential for bias (randomization and blinding) and that allow for comparison between intervention groups and control groups (no intervention). A randomized controlled trial is an experiment and can provide sound evidence of cause and effect. A RCT randomly assigns the exposures and then follows patients forward to an outcome.
Cohort studies take a large population who are already taking a particular treatment or have an exposure, follow them forward over time, and then compare them for outcomes with a similar group that has not been affected by the treatment of expsoure being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study. A cohort study starts with the exposure and follows patients forward to an outcome.
Case control studies are studies in which patients who already have a specific condition are compared with people who do not have the condition. The researcher looks back to identify factors or exposures that might be associated with the illness. They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean that one factor necessarily caused the other. A case control study starts with patients who have the outcome and looks backwards to possible exposures.
Case series/Case reports consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have no statistical validity.
Strengths and pitfalls of cohort studies, case-control studies and randomized controlled trials. NLM/NICHSR Introduction to Health Services Research: A Self-Study Course.
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