The examination of credibility effects in predicting achievement, an important step in the study of source credibility effects on attributions, substantiates Birnbaum's findings that variation in the credibility of information can be represented by changes in the weight of the information. Undergraduate subjects (N=65) predicted the performance of hypothetical students on a comprehensive college final exam of medium difficulty, based on IQ scores and study time efforts ratings of varying reliability. The reliability of effort and ability (IQ) information was manipulated to test averaging and multiplying models for differences in prediction. Results indicated that increased reliability for either ability or effort information had greater effects on judged performance. Increased reliability of one type of information lessened the effect of the other type of information. The findings are consistent with an averaging model in which reliability of information influences its weight, but are inconsistent with a multiplying model. (NRB)