Levels of Evidence
Different study designs present higher level evidence depending on the type of clinical question to be answered. The Centre for Evidence-Based Medicine offers a framework with which to evaluate levels of evidence.
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Study Designs
Case series
A report on a series of patients with an outcome of interest. No control group is involved.
Case-Control Study
Case-control studies begin with the outcomes and do not follow people over time. Researchers choose people with a particular result (the cases) and interview the groups or check their records to ascertain what different experiences they had. They compare the odds of having an experience with the outcome to the odds of having an experience without the outcome.
Cross-sectional study
The observation of a defined population at a single point in time or time interval. Exposure and outcome are determined simultaneously.
Cohort Study (Prospective Observational Study)
A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition.
Controlled Clinical Trial
A type of clinical trial comparing the effectiveness of one medication or treatment with the effectiveness of another medication or treatment. In many controlled trials, the other treatment is a placebo (inactive substance) and is considered the "control."
Randomized Controlled Trial
A controlled clinical trial that randomly (by chance) assigns participants to two or more groups. There are various methods to randomize study participants to their groups.
Systematic Review
A summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. The systematic review may also include a quantitative pooling of data, called a meta-analysis.
Meta-Analysis
A
way of combining data from many different research studies. A
meta-analysis is a statistical process that combines the findings from
individual studies.
Study Designs. (2007, November 3.) In NICHSR Introduction to Health Services Research: a Self-Study Course. Retrieved November 3, 2008 from http://www.nlm.nih.gov/nichsr/ihcm/06studies/studies03.html
Glossary. (2005, May 23.) In AHRQ Effective Health Care. Retrieved November 3, 2008 from http://effectivehealthcare.ahrq.gov/tools.cfm?tooltype=glossaryα=A
Glossary of EBM Terms. Retrieved November 3, 2008 from http://www.cebm.utoronto.ca/glossary/index.htm#top
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