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Evidence-Based Research

Study Design Types

Study Design Types


Systematic Reviews usually focuses on a specific clinical question and conducts an extensive literature search to identify studies with sound methodology. The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question.


A Meta-analysis takes a systematic review one step further by combining all the results using accepted statistical methodology.

Randomized, controlled clinical trials. A prospective, analytical, experimental study using primary data generated in the clinical environment. Individuals similar at the beginning are randomly allocated to two or more groups (treatment and control) and the outcomes of the groups are compared after sufficient follow-up time.


A study that shows the efficacy of a diagnostic test is called a prospective, blind comparison to a gold standard study. This is a controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests -- the test under investigation and the "gold standard" test -- to all of the patients in the study.


Cohort studies identify a large population who already has a specific exposure or treatment, follows them over time (prospective), and compares outcomes with another group that has not been affected by the exposure or treatment 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.


Case control studies are studies in which patients who already have a specific condition or outcome are compared with people who do not. Researchers look back in time (retrospective) to identify possible exposures. 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 than one factor necessarily caused the other.


Case series and 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.


From: Duke University Medical Center Library, December 2005 (last retrieved January 20, 2009)


Databases for Evidence-Based Research

Search strategies will vary, depending on the database you use. Click the "Info" icon for suggested search strategies for a database. 

Study Design Terminology

  • Control Group - the group that does not receive the treatment, etc.
  • Randomization - assigns participants by chance to either the treatment group or the control group.
  • Blinding - the participants and/or some of the researchers do not know the groups to which the participants have been assigned.
  • Loss to Follow-up/Intention to Treat - all of the participants should be accounted for even if they did not fully participate.
  • P Value - the probability of any observed differences having happened by chance.  The convention is that .05 is required.
  • Power - the likelihood that a study will detect a statistically significant difference.  The convention is that a power of at least 80% is required.


M - Methodology 

O - Oriented

L - Locators for

E - Evidence-based

S - Searching

The type of question you want to answer will determine the methodology you will need to try and find. Consider using the linked planning sheet created by Leonard E. Gibbs from his book Evidence-based practice for the helping professions: A practical guide with integrated multimedia.