Masked visual analysis: Minimizing type I error in visually guided single-case design for communication disorders

Purpose: Single-case experimental designs are widely used to study interventions for communication disorders. Traditionally, single-case experiments follow a responseguided approach, where design decisions during the study are based on participants’ observed patterns of behavior. However, this approach has been criticized for its high rate of Type I error. In masked visual analysis (MVA), responseguided decisions are made by a researcher who is blinded to participants’ identities and treatment assignments. MVA also makes it possible to conduct a hypothesis test assessing the significance of treatment effects. Method: This tutorial describes the principles of MVA including both how experiments can be set up and how results can be used for hypothesis testing. We then report a case study showing how MVA was deployed in a multiplebaseline across-subjects study investigating treatment for residual errors affecting rhotics. Strengths and weaknesses of MVA are discussed. Conclusions: Given their important role in the evidence base that informs clinical decision making, it is critical for singlecase experimental studies to be conducted in a way that allows researchers to draw valid inferences. As a method that can increase the rigor of single-case studies while preserving the benefits of a response-guided approach MVA warrants expanded attention from researchers in communication disorders.