The reliability and validity of weighted composite scores

The scores on 2 distinct tests (e.g., essay and objective) are often combined to create a composite score, which is used to make decisions. The validity of the observed composite can sometimes be evaluated relative to an external criterion. However, in cases where no criterion is available, the observed composite has generally been evaluated in terms of its reliability. The analyses in this article are based on a simple, content-based model for the validity of the observed composite as an estimate of a target composite, based on a priori weights for the 2 tests. The results suggest that giving extra weight to the more reliable of the 2 observed scores tends to improve the reliability of the composite, and up to a point tends to improve its validity. Giving too much weight to the more reliable score can decrease the validity of the observed composite as a measure of the target composite.