Factors affecting the slope of empirical ROC curves: Comparison of binary and rating responses

Receiver operating characteristics (ROC curves) were obtained at six signal-to-noise ratios, with two response procedures, in each of three detection paradigms. In all three paradigms-one-interval and two-interval simple detection, and two-interval pedestal-a binary response produced ROC curves with slopes near unity at all signal levels, and a category-rating response led to ROC curves whose slopes decreased with increasing signal strength. This result is thought to be attributable to the varying a priori probability of signal presentation associated with our binary response procedure. The index of detectability taken from the negative diagonal of the ROC space was found to be invariant despite the differences in slope.