The point of subjective equality as a tool for accurate and robust analysis in categorization tasks

Categorization studies, in which stimuli vary along a category continuum, are becoming increasingly popular in psychological science. These studies demonstrate the effect of category ambiguity on various behavioral and neural measures. In such studies, researchers manipulate objective category levels by varying the physical properties of the stimuli, and then use these levels as predictors of behavior—assuming they map directly onto participants’ perceived locations along the category continuum. This approach might not be optimal, considering the variability in participants’ category boundary locations (their point of subjective equality, or PSE). In this tutorial, we propose addressing this issue by estimating participants’ individual points of subjective equality, adjusting category levels relative to these points, and conducting statistical analyses on the subjective category levels. Implementing this method significantly improves the statistical power of the analysis in both experimental and simulated data. Adjusting stimulus levels by the points of subjective equality is highly suited for social categorization studies, in which points of subjective equality vary significantly. On a broader scale, it can be applied to a variety of categorization, discrimination, and decision-making studies.