Predicting Lockean from gradational accuracy

The debate about which scoring rule best measures the accuracy of our credences has largely been conducted on an a priori basis. We pursue an empirical approach, asking which rule best predicts a practical, decision-relevant criterion: Lockean accuracy, the ability to make correct categorical judgments based on a threshold of belief. Analyzing a large dataset of probability judgments, we compare the most widely used scoring rules (Brier, logarithmic, spherical, absolute error, and power rules) and find that, among them, there is no single best one. Instead, the optimal choice is context dependent: the Spherical score is the best predictor for lower belief thresholds, while the Power³ rule is best at higher thresholds. In particular, the widely used Brier and log scores are rarely optimal for this task. A mediation analysis reveals that while much of a rule’s success is explained by its ability to reward calibration and sharpness, the Spherical and Brier rules retain significant predictive power independently of these standard virtues.