A credibility statement is a rhetorical device that establishes the validity of the rhetor's position as articulated in a given speech or artifact. Credibility statements are often associated with Aristotelian models of argumentation.
In rhetoric, and specifically argumentation, the credibility of a speaker can be considered to be a signifier of the validity of their position. A credibility statement is a rhetorical device which seeks to establish this validity by exemplifying the qualities of the speaker. Following an Aristotelian model of rhetoric, this is typically achieved through an appeal to ethos, or character.
In the Aristotelian model, rhetoric functions along the axes of the Artistic Proofs: Logos, pathos, and ethos. These proofs represent, to paraphrase James Herrick from The History and Theory of Rhetoric, logical argumentation, emotional psychology, and the sociology of good character, accordingly.
As it can be seen, while the first proofs concern themselves primarily with the articulation of the speech itself, ethos concerns itself with the character of the rhetor himself. The importance of character in argumentation is found in the assumption that personal morality influences the explicit value of other statements, be they related or unrelated to that specific character quality.
To that end, a credibility statement specifically addresses the speaker's social context such as to establish the validity of their position on the matter at hand. Be it through the development of empathy via shared experience, the statement of expertise through practice and history, or the establishment of intent through the detailing of past intention, credibility statements support the argument of the speaker by supporting the speaker him or her self in the eyes of the audience.