First, Slavin advanced the ―Principle of Best Evidence‖ which requires that the best available evidence, in the form of rigorously designed relevant empirical studies be included in the review at the expense of less rigorously designed studies; thus, overcoming researcher bias and subjectivity, provided that explicit study inclusion criteria were included in the bestevidence synthesis report. Second, Slavin (1986, pp. 7-8) presented four standards for the explicit statement of study inclusion criteria:
255 a. The study must be germane to the problem or issue under investigation. b. The study's methodological rigor must be analyzed and found to be suitable and to have minimized as much as possible, bias. c. Study external validity should be as highly valued as its internal validity. d. As appropriate, an explanation of the rationale for excluding some studies should be presented, thus giving the reader a full picture of the inclusion criteria application. 3. Slavin argued for the inclusion of unpublished studies such as dissertations and government or foundation reports, provided they were well designed and germane to the topic under consideration. Slavin also argued that rigorous studies, where effect size could not be or were not computed, be included, to provide additional context or explanatory information, provided such studies were well designed. In pooling effect sizes, Slavin