The state of the science in the meta-analysis of single-case experimental designs

The articles in the previous, special issue of Evidence-Based Communication Assessment and Intervention provided an excellent review of the meta-analysis of single-case designs. This article weaves commentary about those articles into a larger narrative about two major lines of attack on this problem: the use of parametric approaches like regression and multilevel modeling, and the development of parametric and nonparametric effect-size estimators. On each of these two topics, we describe an agenda of research topics that need to be addressed; and we also introduce a new effect-size estimator that may prove to be comparable to the usual standardized mean difference statistics (d) widely used in between-groups analysis. The article ends with observations about ways in which developments in the meta-analysis of single-case designs may have far wider implications than previously appreciated.