EBP- Study Material (4)Strength of Meta-analyses (Level I Evidence) The strength of the evidence on which recommendations are made within a meta-analytic design depends on the type and quality of studies included in the meta-analysis as well the design of the meta-analysis itself. Factors to take into consideration include sampling criteria (primary studies included in the analysis), quality of the primary studies, and statistical heterogeneity (variability) of results across studies (Polit & Beck, 2008). First, the EBP team needs to lookat the types of research designs (level of evidence) included in the meta-analysis. The strongest meta-analyses contain only randomized controlled trials. Evidence from these studies is level I evidence. Some meta-analyses include data from quasi-experimental or non-experimental studies; hence, evidence would be a level commensurate with the lowest level of research design included (e.g., if the meta-analysis included experimental and quasi-experimental studies, then the meta-analysis would be level II).