An audience has every right to ask a speaker, "How do you know?" Supporting material is the answer. It turns claims into something listeners can weigh, and it is also how a speaker earns the right to be believed.

Examples

Examples make ideas concrete. A brief example is a quick illustration; an extended example is a story told in some detail. A hypothetical example describes an imagined situation, such as "Suppose you came home to find your street flooded." Hypotheticals are useful for making listeners picture something, but they must be clearly labeled as imagined. Presenting an invented story as real is a form of lying, however good the cause.

Statistics

Numbers can be powerful and are easily lost by the ear. Round off complicated figures and explain what they mean: "about one in four" sticks better than "24.7 percent." Compare big numbers to something familiar, such as how many stadiums they would fill. Use few of them, give their source and date, and make sure they measure what you claim they measure. Numbers mislead easily; a speaker's duty is to avoid passing those errors on.

Testimony

Testimony is quoting or paraphrasing others. Expert testimony comes from people recognized as authorities in a field. Peer testimony comes from ordinary people with firsthand experience, such as a nurse describing a night shift or a tenant describing a winter without heat. Each has its place. Experts lend authority; peers lend reality. Quote experts only on matters within their expertise.

Citing sources aloud

Listeners cannot see footnotes, so a speaker gives an oral citation: naming the source aloud as you use it. "According to a 2025 report by the state health department" takes four seconds and tells the audience who said it and when. If the source is not well known, add a few words on why it is qualified. Presenting someone else's words or ideas as your own is plagiarism in a speech just as in an essay.

Testing your evidence

Before using any piece of support, ask whether it is accurate, recent enough, relevant to the exact point, and from a source with no hidden stake in the answer. Ask whether one vivid example is typical or an exception.

Checking the record

Even famous speeches need checking. In 1851 Sojourner Truth spoke at a women's rights convention in Akron, Ohio. A few weeks later, her friend Marius Robinson printed an account in a newspaper, the Anti-Slavery Bugle, in standard English, including the line "I have as much muscle as any man, and can do as much work as any man." The famous version with the refrain "Ar'n't I a woman?" was published by Frances Dana Gage in 1863, twelve years later, and put Truth's words into a heavy Southern dialect, although Truth grew up in New York speaking Dutch as her first language. Historians generally treat the Robinson report as closer to what was said. The rule for any disputed text: prefer the earliest source closest to the event.

Credibility

Listeners judge not only the evidence but the speaker: whether you seem to know the subject and whether you can be trusted. The idea goes back to ancient Greece. Modern speech textbooks divide credibility by timing. Initial credibility is what listeners think of you before you begin. Derived credibility is what you earn or lose during the speech. Terminal credibility is the impression left when you finish. A little-known speaker can start low and end high.

You build credibility by mentioning relevant experience briefly, showing that you have done the research, being fair to people who disagree, and admitting what you do not know. Research on two-sided messages suggests that naming the other side's case and answering it usually persuades better than ignoring it, at least with audiences who will hear that case anyway.

Worked example: a lawyer from the prairie

On 27 February 1860, Abraham Lincoln, then known mainly as an Illinois lawyer who had lost a Senate race, spoke at the Cooper Institute in New York. The audience of eastern Republicans was sophisticated and doubtful. His initial credibility with them was modest.

His opponent Stephen Douglas had claimed that the founders understood the slavery question better than anyone now living, and that they had denied the federal government power over slavery in the territories. Lincoln accepted the first part and tested the second. He took the thirty-nine men who signed the Constitution and, after weeks of research in records of congressional votes, traced how each had acted when the question came before them. He reported that a clear majority, twenty-one of the thirty-nine, had acted in ways showing they thought the federal government could control slavery in the territories.

The evidence was dry and devastating. Lincoln's derived credibility rose with every example; listeners saw a man who had done the work. He ended with a line still quoted: "Let us have faith that right makes might, and in that faith, let us, to the end, dare to do our duty as we understand it." The speech was printed and widely circulated, and it helped make him a serious candidate for the presidency that year.