Often called ethos in the world of the rhetorician, source credibility holds a place of dire importance in understanding how a message will be received. Simply put, a person with more knowledge on a topic is more credible. Our society is generally skeptical and we question the source of statistics and ideas particularly when they conflict with what was already rooted in our head.
Hovland and Weiss conducted a study using written messages and varied the source to which the same message was attributed among different receivers. A more credible individual or institution had greater power to change attitudes than a source determined less credible.
Yet, this dimension of effects work is not unilateral. Studies developed many different aspects of credibility denying that absolute power of a source to be credible or not. Markham used scales to rate the contributing factors of credibility and determined three major indicators that he labeled as reliable-logical, showmanship, and trustworthiness. (Bettinghaus p. 92)
Further study was carried out by Osgood and Tannenbaum who looked at ratings along polar continuums such as good-bad and competent-incompetent.
All of the research on this topic clearly points to two major factors often attributed as trustworthiness and competency. Other factors have been uncovered such as composure, sociability, and extroversion which can be summed together under personal characteristics. (Bettinghaus p. 92)
Despite some debate over the actual components of a source's credibility, consensus does indicate that the perceived credibility of a source makes a significant difference in the acceptance of new attitudes among an audience.
Finally, a phenomenon labeled the "sleeper effect" has shown that the receiver actually can disassociate the message from the source. Thus, a message previously thought to have come from a disreputable source may have the ability to mold convictions once that source association is forgotten.