The What Works Association has developed a methodology for assessing strength of evidence, called the modified Gray Scale. This six-level scale, modified from the original five levels developed by Sir Muir Gray, founder of the Cochrane Collection, helps summarize the types of studies supporting a finding. To assess the strength of evidence of a body of evidence, criteria are set for the number of studies, the number of countries, and the Gray Scale ratings for each study.
Sources: Gray, JA, & Chambers, LW. 1997. Evidence-based Health Care: How to Make Health Policy and Management Decisions. Canadian Med Assoc Jnl, 157(11); Gray, JAM, 2009. Evidence-Based Health Care and Public Health. 3rd ed., Elsevier Health Sciences, Oxford, 2009; Gay, J. Croce-Galis, M & Hardee, K. 2016. What Works for Women and Girls: Evidence for HIV/AIDS Interventions. NY: Population Council, Evidence Project and What Works Association, Inc.
The below projects and publications have used this scale and strength of evidence methodology:
The methodology section of whatworksforwomen.org provides an in-depth example of using this modified gray scale to assess evidence for programs.