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Social Work: Evidence-Based Practice

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Forms of Evidence

This guide is designed to give you tips for finding evidence-based sources for your research.

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. In this page, you will see an evidence hierarchy outlined by Nathan and Gorman (2015).

In the Library

An Evidence Hierarchy

In 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. (2007). A guide to treatments that work. Oxford: Oxford University Press.