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Research Methods


Forms 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. Nathan and Gorman (2015) outline a useful evidence hierarchy.

In the Library

An Evidence Hierarchy

In the preface to A Guide to Treatments that Work, 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 include randomized clinical trials.
  • Type 2 studies include clinical trials, but some aspects of the type 1 study may be missing (for example, the assignment may not be randomized).
  • Type 3 studies 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 are reviews with secondary data analysis. This can include a meta-analysis.
  • Type 5 studies are reviews without secondary data analysis.  This can include literature reviews.
  • Type 6 studies include case studies, essays, and opinion papers.

Nathan, P.E. & Gorman, J.M. (2015). A guide to treatments that work. Oxford: Oxford University Press.