Believability is a measure of your willingness to accept as true the reasons and support the writer gives in defense of a thesis.

To test for believability, ask: On what basis am I being asked to believe this reason or support is true? If it cannot be proved true or false, how much weight does it carry?

In judging facts, examples and anecdotes, statistics, and authorities, consider the following points.

Facts are statements that can be proved objectively to be true. The believability of facts depends on their accuracy (they should not distort or misrepresent reality), their completeness (they should not omit important details), and the trustworthiness of their sources (sources should be qualified and unbiased). King, for instance, asserts as fact that the African American will not wait much longer for racial justice (par. 8). His critics might question the factuality of this assertion by asking: Is it true of all African Americans? How does King know what African Americans will and will not do?

Examples and anecdotes are particular instances that may or may not make you believe a general statement. The believability of examples depends on their representativeness (whether they are truly typical and thus generalizable) and their specificity (whether particular details make them seem true to life). Even if a vivid example or gripping anecdote does not convince readers, it usually strengthens argumentative writing by clarifying the meaning and dramatizing the point. In paragraph 5 of the King excerpt, for example, King supports his generalization that some African American extremists are motivated by bitterness and hatred by citing the specific example of Elijah Muhammad’s Black Muslim movement. Conversely, in paragraph 9, he refers to Jesus, Paul, Luther, and others as examples of extremists motivated by love and Christianity. These examples support his assertion that extremism is not in itself wrong and that any judgment of extremism must be based on its motivation and cause.

Statistics are numerical data. The believability of statistics depends on the comparability of the data (the price of apples in 1985 cannot be compared with the price of apples in 2012 unless the figures are adjusted to account for inflation), the precision of the methods employed to gather and analyze data (representative samples should be used and variables accounted for), and the trustworthiness of the sources.

Authorities are people to whom the writer attributes expertise on a given subject. Not only must such authorities be appropriate, as mentioned earlier, but they must be credible as well—that is, the reader must accept them as experts on the topic at hand. King cites authorities repeatedly throughout his essay. He refers to religious leaders (Jesus and Luther) as well as to American political leaders (Lincoln and Jefferson). These figures are likely to have a high degree of credibility among King’s readers.