Three-Sided Testing to Establish Practical Significance: A Tutorial

Researchers may want to know whether an observed statistical relationship is either meaningfully negative, meaningfully positive, or small enough to be considered practically equivalent to zero. Such a question cannot be addressed with standard null hypothesis significance testing or standard equivalence testing. Three-sided testing (TST) is a procedure to address such questions by simultaneously testing whether a relationship is significantly bounded below, within, or above predetermined smallest effect sizes of interest. TST is a natural extension of the standard procedure of two one-sided tests (TOST) for equivalence testing. TST offers a more comprehensive decision framework than TOST with no penalty to error rates or statistical power. In this article, we give a nontechnical introduction to TST; provide commands for conducting TST in R, Jamovi, and Stata; and provide a Shiny app for easy implementation. Whenever a meaningful smallest effect size of interest can be specified, TST should be combined with null hypothesis significance testing as a standard frequentist testing procedure.