A note on aggregating opinions

This note examines the number of experts to be included in a prediction group where the criterion of predictive ability is the correlation between the uncertain event and the mean judgment of the group members. It is shown that groups containing between 8 and 12 members have predictive ability close to the “optimum” under a wide range of circumstances but provided (1) mean intercorrelation of experts' opinions is not low (<.3, approximately) and/or (2) mean expert validity does not exceed mean intercorrelation. Evidence indicates these exceptions will not be common in practice. The characteristics needed by an additional expert to increase the validity of an existing group are also derived.