The What Works Clearinghouse (WWC) Standards Briefs explain the rules the WWC uses to evaluate the quality of studies for practitioners, researchers, and policymakers. This brief explains what baseline equivalence is and why it matters. As part of the WWC review process for certain types of studies, reviewers assess whether the intervention group…

This paper reviews limitations in the interpretations of group differences in studies that use younger, language-matched controls in addition to age-matched controls to examine the role of "language level" on performance. Problems arise because of the multidimensional nature of language, extraneous age effects associated with language matching,…

A meta-analysis was conducted on 44 educational studies that used either a (labelled) Hawthorne control group, a manipulation of Hawthorne effects, or a group designed to control for the Hawthorne effect. The sample included published journal articles, ERIC documents or unpublished papers, and dissertations. The studies were coded on 20 variables,…

This article discusses research design problems peculiar to work with intellectually disabled subjects, such as discrepancies in uses of control groups and mental age matching, definitions of subject groups and variability within and between subject groups. Suggestions for minimizing these problems are made and alternative methodologies offered.…

Ninety-two research reports involving specifically language-impaired subjects were reviewed. Analysis showed few consistent trends with respect to subject selection, subject description, and the number and types of control groups. The importance of more complete subject descriptions and matching criteria for control groups is discussed.…