Signal Detection Theory (SDT) assumes that responses are governed by perceptual noise and a flexible decision criterion, but recent criticisms suggest that these assumptions fundamentally misrepresent perceptual and decision processes (J. D. Balakrishnan, 1999). We hypothesize that these findings stem from decision noise: the inability to use deterministic response criteria. To test this hypothesis, we present a simple extension of SDT (the Decision Noise Model) and a new measure of perceptual noise that together help determine the locus of violations of SDT. Results from a new experiment provide unambiguous support for the decision noise hypothesis, and show that confidence ratings are especially unreliable and inconsistent. In addition, the Decision Noise Model successfully accounts for our own data as well as those from previously published studies. These findings suggest that decision noise may be important across a wide range of tasks and needs to be understood in order to accurately measure perceptual processes. |