Marketing Letters 10:1 (1999): 51–62 © 1999 Kluwer Academic Publishers, Manufactured in The Netherlands The Moderating Impact of Quantitative Information on the Relationship Between Source Credibility and Persuasion: A Persuasion Knowledge Model Interpretation NANCY ARTZ University of Southern Maine ALICE M. TYBOUT Northwestern University Abstract An experiment manipulated source expertise, source bias, and message format. The findings reveal that expert sources are expected to quantify message claims whereas non-expert sources are not. Persuasion is greater when these expectations are met versus when the source and the message format are incongruent, but only when the source also has self-interest in the advocacy. It appears that source-message incongruity and source bias focus attention on the source and, in combination, lead to negative inferences about the source’s manipulative intent. This interpretation is consistent with the Persuasion Knowledge Model (Friestad and Wright 1994). Key words: Source expertise, source bias, quantitative claims, attitude change Introduction Intuition suggests that increasing the credibility of the person who delivers the message will increase persuasion. However, a growing body of research reveals that this seemingly self-evident proposition does not always hold. Characteristics of the message can influ- ence whether source credibility enhances, undermines, or has no effect on persuasion. Two studies are of particular interest because they offer different explanations for the variety of effects associated with source credibility.We begin by reviewing these studies and then turn to an experiment designed to shed light on the underlying process. 1. The relationship between source credibility and persuasion Yalch and Elmore-Yalch (1984) examined the effects of source expertise and whether message claims were quantified (e.g. “...many people do 95% of their banking using automatic teller machines” versus “...many persons do virtually all their banking using Kluwer Journal @ats-ss11/data11/kluwer/journals/mark/v10n1art4 COMPOSED: 11/20/98 8:42 am. PG.POS. 1 SESSION: 13