The relative psychometric function: A general analysis framework for relating psychological processes

Psychophysics seeks to quantitatively characterize relationships between objective properties of the world and subjective properties of perception. However, traditional approaches fall short of identifying quantitative relationships among psychological processes. This latter goal is particularly important when certain processes may depend on each other, such as for subjective experience and task performance: Typically, better performance is accompanied by stronger subjective experiences of clarity, vividness, or confidence. But is the relationship between performance and subjective experience fixed, or can it vary, for example, by task or attentional demands? Such questions are key for better understanding psychological processes in general and subjective experience in particular. Here, we develop and showcase a new psychophysical method designed to answer such questions: relative psychometric function (RPF) analysis, which characterizes the nonlinear psychometric relationships between psychological processes and how these relationships change under different circumstances (e.g., experimental manipulations). We demonstrate the advantages of RPF analysis using a data set in which human subjects discriminated random dot kinematograms which varied in motion coherence and overall dot density (dots per visual degree) and rated confidence. RPF analysis revealed systematic changes in the relationship between performance and two subjective measures (confidence and metacognitive sensitivity) due to dot density and task design choices. These empirical results showcase how RPF analysis may reveal changes in quantitative relationships between any two psychological measures in future studies: performance, vividness, clarity, reaction time, confidence, and more. To enable the scientific community to use RPF analysis on their data, we also present our open-source RPF toolbox. (PsycInfo Database Record (c) 2026 APA, all rights reserved).