Single-case research design is a useful methodology for evaluating the presence of a functional relation between an intervention and the mathematical performance of students with a learning disability. However, a functional relation cannot be established with threats to internal validity of the design. External validity is impacted if researchers do not clearly describe their methods so that others may replicate them. Therefore, single-case researchers must maximize the internal and external validity of their investigations. We provide a commentary on investigations published in "Learning Disability Quarterly" that implemented a mathematics intervention for students with a mathematics learning disability. We will highlight specific features of single-case designs within the review and provide recommendations for the field.