Case Series and Case Reports:
- Collections of reports on the treatment of individual patients or a report on a single patient.
- No control groups with which to compare outcomes, so limited statistical validity.
Case Control Studies:
- Patients who already have a specific condition are compared with people without the condition. Researcher looks back to identify factors or exposures possibly associated with the condition, often relying on medical records and patient recall.
- Less reliable because showing a statistical relationship does not mean than one factor necessarily caused the other.
- Starts with patients who already have the outcome and looks backwards to possible exposures.
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 for outcomes with a similar group that has not been affected by the treatment or exposure.
- Observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.
- Starts with the exposure and follows patients forward to an outcome.
Randomized, Controlled Clinical Trials:
- Carefully planned projects that introduce a treatment or exposure to study its effect on patients.
- Include methodologies that reduce the potential for bias (randomization and blinding) and allow for comparison between intervention and control groups.
- Is an experiment and can provide sound evidence of cause and effect.
- Randomly assigns exposures and then follows patients forward to an outcome.
Systematic Reviews:
- Usually focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify studies with sound methodology.
- The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question.
Meta-Analysis:
- Thoroughly examines a number of valid studies on a topic and combines 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.
