The Effects of Non-Diagnostic Information on Confidence and Decision Making.

Many decision-making tasks are characterized by a combination of diagnostic and non-diagnostic information, yet models of responding and confidence almost exclusively focus on the contribution of diagnostic information (e.g., evidence associated with stimulus discriminability), largely ignoring the contribution of non-diagnostic information. An exception, Baranski and Petrusic’s (1998) doubt-scaling model, predicts a negative relationship between non-diagnostic information and confidence, and between non-diagnostic information and accuracy. In two perceptual-choice tasks, we tested the effects of manipulating non-diagnostic information on confidence, accuracy, and reaction time (RT). In Experiment 1 (N=56), participants viewed a dynamic grid consisting of flashing blue, orange and white pixels and indicated whether the stimulus was predominantly blue or orange (using a response scale ranging from low confidence blue to high confidence orange), with the white pixels constituting non-diagnostic information. Increasing non-diagnostic information reduced both confidence and accuracy, generally slowed RTs, and led to an increase in the speed of errors. Experiment 2 (N=20) was a near exact replication of Experiment 1, however this time participants were not asked to provide a confidence rating. This was to determine whether asking participants to make a decision and provide a confidence rating simultaneously influenced choosing behaviour. Like the first experiment, Experiment 2 found that increasing non-diagnostic information reduced both accuracy and generally slowed RTs (with an increase in the speed of errors), providing further support for the doubt-scaling model of confidence.