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Social Work 8952: Evidence Based Practice: Gold Standard of Evidence

Forms of Evidence

Research studies come in many different forms. Experts in the field of evidence-based social work practice often rank types of research studies according to their reliability. These rankings often take the form of an an evidence hierarchy. Below you will see an evidence hierarchy outlined by Nathan and Gorman (2007).

An Evidence Hierarchy

Nathan and Gorman outline a hierarchy of research articles. Articles based on type 1 studies represent the most reliable form of evidence. 

  • Type 1 Studies: these include randomized clinical trials.
  • Type 2 Studies: these include clinical trials, but some apects of the type 1study may be missing (for example, the assignment may not be randomized).
  • Type 3 Studies: these are open treatment studies aimed at getting pilot data. These are subject to bias. Type 3 studies can include case controlled studies.
  • Type 4 Studies: type four studies are reviews with secondary data analysis. This can include a meta-analysis.
  • Type 5 Studies: reviews without secondary data analysis.
  • Type 6 Studies: these include case studies, essays, and opinion papers.

Nathan, P.E. & Gorman, J.M. (2007). A Guide to treatments that work. Oxford: Oxford University Press.

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.


Retrieved from Duke University Medical Center Library

Evaluating the Quality of Evidence