Dilution and confirmation of probability judgments based on nondiagnostic evidence

Previous research has shown that probability judgments based on a mix of diagnostic and nondiagnostic information are less extreme than judgments based on the diagnostic information alone. Results of the present experiments suggest that this dilution effect holds only under a limited set of conditions. When judgments based on a mix of diagnostic and nondiagnostic information are compared with separately elicited judgments based on the diagnostic information alone, the dilution effect is consistently observed. When judgments based on the diagnostic evidence are revised in light of additional, nondiagnostic evidence, by contrast, the dilution effect is eliminated or even reversed (yielding a confirmation effect) depending on the type of nondiagnostic evidence under evaluation.