Selecting the Proper Tau-U Measure for Single-Case Experimental Designs: Development and Application of a Decision Flowchart

A variety of measures have been developed to quantify intervention effects for single-case experimental design studies. Within the family of non-overlap indices, the Tau-U measure is one of the most popular indices. There are several Tau-U variants, each one calculated differently. The appropriateness of each Tau-U variant depends upon the data characteristics present within the study (e.g. number of measurement occasions, the within-case variability, and baseline trend). However, inconsistent terminology is used to refer to the Tau-U variants, and researchers can overlook the attributes of the different Tau-U variants. As a result, the Tau-U variants can be applied inappropriately, and this can result in invalid conclusions of intervention effectiveness. This paper proposes a Tau-U flowchart that can assist the decision-making process when using Tau-U with single-case experimental designs that incorporate baseline-intervention (AB) comparisons (e.g. multiple-baseline designs, withdrawal/reversal designs, etc.). The flowchart can help researchers select the appropriate Tau-U variant to use based on their data and research questions. The flowchart is applied to two single-case experimental studies to demonstrate its use.