What Works Clearinghouse Standards and Generalization of Single-Case Design Evidence

A recent review of existing rubrics designed to help researchers evaluate the internal and external validity of single-case design (SCD) studies found that the various options yield consistent results when examining causal arguments. The authors of the review, however, noted considerable differences across the rubrics when addressing the generalization of findings. One critical finding is that the What Works Clearinghouse (WWC) review process does not capture details needed for report readers to evaluate generalization. This conclusion is reasonable if considering only the WWC's SCD design standards. It is important to note that these standards are not used in isolation, and thus generalization details cannot be fully understood without also considering the review protocols and a tool called the WWC SCD review guide. Our purpose in this commentary is to clarify how the WWC review procedures gather information on generalization criteria and to describe a threshold for judging how much evidence is available. It is important to clarify how the system works so that the SCD research community understands the standards, which in turn might facilitate use of future WWC reports and possibly influence both the conduct and the reporting of SCD studies.