Development of a Distance-Based Effect Size Metric for Single-Case Research: Ratio of Distances

This article describes the development of an effect size measure called Ratio of Distances (RD). The goal was to develop a measure of level change for single case experimental research that met several practical requirements: (a) the measure is adaptable to designs with varying numbers of observations per, and across, phases; (b) the measure is adaptable to situations in which slope does and does not exist; (c) the measure has no ceiling, as is the limitation with commonly used overlap-based measures of effect size; and (d) the measure is computationally transparent and easily computed using widely available analysis tools (e.g., Microsoft Excel). The measure is applicable to single cases and meta-analyses. •New effect size measure for single-case experimental designs•Distance-based metric, comparable to measures used in other areas of psychology•Promotes inclusion of single-case data in meta-analyses•Applicable to all pairwise comparisons•Can be detrended for isolation of level change across phases