The purpose of the present study was to provide the means to evaluate the "interval-scaling" assumption that governs the use of parametric statistics and continuous data estimators in self-report instruments that utilize Likert-type scaling. Using simulated and real data, the methodology to test for this important assumption is evaluated using the popular software Mplus 8.8. Evidence on meeting the assumption is provided using the Wald test and the equidistant index. It is suggested that routine evaluations of self-report instruments engage the present methodology so that the most appropriate estimator will be implemented when testing the construct validity of self-report instruments.