Initially excluded from many evaluations of education research, single-case designs have recently received wider acceptance within and beyond special education. The growing approval of single-case design has coincided with an increasing departure from convention, such as the visual analysis of results, and the emphasis on effect sizes comparable with those associated with group designs. The use of design-comparable effect sizes by the What Works Clearinghouse has potential implications for the experimental literature in special education, which is largely composed of single-case designs that may not meet the assumptions required for statistical analysis. This study examined the compatibility of single-case design studies appearing in 33 special education journals with the design-comparable effect sizes and related assumptions described by the What Works Clearinghouse. Of the 1,425 randomly selected single-case design articles published from 1999 to 2021, 59.88% did not satisfy assumptions related to design, number of participants, or treatment replications. The rejection rate varied based on journal emphasis, with publications dedicated to students with developmental disabilities losing the largest proportion of articles. A description of the results follows a discussion of the implications for the interpretation of the evidence base. (PsycInfo Database Record (c) 2024 APA, all rights reserved).