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