Small group predictions on an uncertain outcome: The effect of nondiagnostic information

Research has established that exposure to a combination of diagnostic (i.e. , relevant) and nondiagnostic (i.e., irrelevant) information results in pre dictions that are more regressive than predictions based on diagnostic info rmation (Hackenbrack, 1992; Hoffman and Patton, 1997). This phenomenon has been labeled the dilution effect (e.g., Tetlock and Boettger, 1989) and has been documented when individuals make predictions. This study tests for th e dilution effect when small groups make predictions, and examines the effe ct of using a procedure designed to reduce the dilution effect. Results ind icate that group predictions are influenced by nondiagnostic information in the same manner as are individual predictions, and allowing participants t o rate the diagnosticity of information prior to making predictions does no t reduce the dilution effect.