Levels of Evidence

It's useful to consider the varying levels of evidence when thinking about the strength of research. The levels in the figure below come from the medical literature - and tend to privilege a positivist research perspective. Notice that the methodological design of the research plays a role in its level - and  more generalizable, quantitative results are privileged. This is one perspective, and it is contested, especially in the social sciences!
From Pinto, L., Spares, S. & Driscoll, L. (2012). 95 Strategies for Remodeling Instruction. Thousand Oaks, CA: Sage.