Webster’s Dictionary defines "credible" as plausible or deserving confidence. (Webber et al., p. 166) Therefore source credibility could be said to be those qualities that determine the total character, trustworthiness and perceived experience of the persuasive message source. An interesting aspect of source credibility is that is determined entirely by the perceptions of the audience receiving the persuasive message. (Nelson and Pearson, p. 253) The message source may attempt to increase their credibility through revealing relevant experience, learned knowledge and expertise, but the final determination of source credibility is up to the audience.

As it relates to the advertising industry, source credibility is a tricky subject. Advertising messages are generated with information gained from the client and passed through the filter of the agency’s account and creative staff. The message is further tweaked and prodded along the way by a myriad of other people including but not limited to agency creative directors, client management officers, spouses, so on and so forth. By the time an advertisement is finally published or broadcast, it is difficult, if not possible for the general public to discern exactly who is responsible for the message.

The general public, being largely unaware of the specifics regarding which advertising agencies service which clients, determines source credibility almost entirely on the basis of brand image. Given the importance that source credibility has in the persuasive process as a whole, agencies literally have their clients’ credibility in their hands when they create advertising messages. This is one of the many reasons that advertising practitioners must have a clear understanding of the power of source credibility.