Supporting your Persuasion: Credibility, Evidence, Logic

“Prove it to me.” When someone makes a claim, you don’t always accept it at face value. Sometimes you want to know more details. You want to know how they reached their conclusions and where they’re getting their evidence from. This evidence, in turn, has to be trustworthy. “I read it on the internet” has become a punchline.

In the same way, your audience wants to know how you arrived at the argument you’re making in your speech. The audience will judge your credibility from the beginning of your speech through the end. Based on the content you deliver, and how you deliver it, they’ll decide if they trust you enough to support your cause.

Establishing Credibility

If you are going to persuade an audience, you have to first get them to trust you and see you as a credible communicator. Being credible as a speaker means showing your audience you have their best interests at heart, that you are knowledgeable about the subject you are speaking about, and that the evidence you use to support your argument is trustworthy.

Communication scholar Stephen Lucas says that speaker credibility is affected most by two factors:

• Competence: How the audience views your intelligence, knowledge, and expertise on the subject you are speaking about.

• Character: How the audience views your concern for them, sincerity, and trustworthiness.

Lucas further advises that speakers do the following to establish their credibility:

• Explain to your audience why you are qualified to speak on the topic. Provide your own personal experience with your topic, if relevant. Demonstrate that you have done sufficient research on the topic to speak about it with authority.

• Establish common ground with your audience. Identify with your audience and show them how the case you are making is consistent with their values and beliefs.

• Deliver your speech fluently, expressively, and with confidence. If you speak with conviction and communicate you care about both your speech topic and your audience, you are more likely to be successful.^[1]

A major part of coming across as a competent and credible speaker involves putting together a well-crafted speech with effective arguments in favor of your proposition.

Evidence in a Persuasive Speech

Part of being perceived by your audience as a credible speaker involves using evidence well suited to the topic of the speech and the audience you are speaking to. Imagine if a speaker made an argument that in 30 days Earth would be invaded by Martians. You’d want to see some credible evidence supporting that claim, right?

Regardless of whether you’re arguing that Martians are going to invade Earth or that Doc Martens are the best shoes, you want to support your claims with helpful, credible, and convincing evidence.

Evidence is necessary to use in a speech because demonstrating that you’ve taken the time to research your topic enhances your credibility as a speaker and adds to the persuasive appeal of your argument. Each main point you make in a persuasive speech should be supported by at least one type of evidence.

Evidence is also crucial when you are speaking to an audience that doesn’t agree with your perspective. Careful use of evidence can help you answer the questions that audience members who don’t agree with you may have about the argument you are making.

There are several sources of evidence that can help establish your credibility as a persuader:

• Testimony from experienced and expert individuals will lend credibility to your speech. If there is an expert on your speech topic on your campus or in your community, you could interview them and use their testimony as supporting evidence in your speech.

• Nonacademic sources, which include books, general interest or trade periodicals, newspapers, blogs, social media sites, and websites like Wikipedia. Nonacademic sources can be a good starting point for research.

• Academic sources, which include books, scholarly articles, journal databases, and web resources. Academic sources are usually peer reviewed by experts and they generally are more specific in focus and tailored to an audience of highly knowledgeable readers than nonacademic sources.

As you are researching, it’s helpful to ask yourself questions such as:

• Am I drawing from a wide enough variety of sources that allows me to see a full range of perspectives on my topic?

• Am I choosing sources that are reputable and credible?

• Do the sources I am choosing have an agenda or bias I need to be aware of? And if I use a source with an agenda or bias, how should I present that information in my speech? Generally audiences will find evidence from a neutral source more persuasive than from a source with a bias or agenda.

• Am I verifying that the information I find through one source is corroborated by other sources? Generally it’s good practice to verify a claim made by one source by checking it against at least one other reputable source.

The Four Moves

A useful tool to assess the quality of your evidence is the Four Moves, which can help you uncover the truth of a claim.

Move 1: Stop

Think critically. Avoid being too emotionally charged or looking for information that confirms your own biases. Have an open mind to consider new or controversial topics and seek to understand. Then ask yourself whether you know and trust the website or source of the information. Don’t use the information in your speech until you know you can trust the source.  Also, as you start the source verification process, try not to get  lost in a “click cycle.” If you feel yourself getting overwhelmed in your fact-checking efforts, STOP and take a second to remind yourself what your goal is. Adjust your strategy if it isn’t working. Make sure you approach the verification process at the right amount of depth for your purpose.

Move 2: Investigate the Source

Find out who the author is, why they wrote it, etc. The key idea of investigating is to know what you’re reading before you read it. For example, if you’re reading a piece on economics by a Nobel prize-winning economist, you should know that before you read it. Conversely, if you’re watching a video on the many benefits of milk consumption that was put out by the dairy industry, you probably want to know that as well.

This doesn’t mean the Nobel economist will always be right and that the dairy industry can’t ever be trusted. But knowing the expertise and agenda of the source is crucial to your interpretation of what they say. Taking sixty seconds to figure out where a source comes from before reading will help you decide if it is worth your time and, if it is, help you to better understand its significance and trustworthiness.

This video shows some really simple ways to do a quick investigation of a source.

You can view the transcript for “Online Verification Skills-Video 2: Investigate the Source” here (opens in new window).

Move 3: Find Better Coverage

Read laterally, which means comparing what your source is saying to what other sources are saying.  You’ll want to know if your source represents a consensus viewpoint, or if it offers information that is the subject of much disagreement. For example, if you find an article that says koalas have just been declared extinct from the Save the Koalas Foundation, then the best strategy would be to find other sources that cover this news to see what the consensus seems to be. In these cases, find trusted coverage that better suits your needs – more trusted, more in-depth, or maybe just more varied. You can also use fact-checking websites such as Snopes (opens in new window) or factcheck (opens in new window) to confirm the truthfulness of claims.

Move 4: Trace claims, quotes, and media to the original source

A lot of things you find, especially in general searches online, have been stripped of context. Maybe there’s a video of a fight between two people. But what happened before the fight? Who started it? What was clipped out of the video and what stayed in? Maybe there’s a picture that seems real but the caption is dubious at best. Maybe a claim is made about a new medical treatment supposedly based on a research paper, but you’re not certain if the paper supports it. In these cases, try to trace the claim, quote, or media back to the source, so you can see it in its original context and get a sense for if the version you saw was accurately presented.

This video gives advice on tracking a claim back to its original source.

You can view the transcript for “Online Verification Skills- Video 3: Find the Original Source” here (opens in new window).

Checking for Logic

All of your support should be logical, that is, reasonable to most people.  Being logical sounds easier than it is.

It’s very easy for logical fallacies, or errors in reasoning, to creep into a persuasive speech. Think of a logical fallacy as a persuasive appeal that contains logical errors or poor use of evidence. You want to avoid fallacies when you speak to an audience, but fallacies are something you should also listen for as an audience member, to determine whether a speaker’s argument is valid. Below is a list of common logical fallacies that you need to avoid in your support for a persuasive speech:

Hasty Generalizations

When you make a hasty generalization, you make the error of jumping to a conclusion based on insufficient evidence.  Most hasty generalizations apply a characteristic to “all” members of a group, often a non-representative sample. For example, if you claimed that “Phoenix is always snowy and cold” based on the evidence of two snowy days in January, you’d be guilty of 1) not basing your claim on a sufficient number of cases (as two days hardly qualifies as sufficient) and 2) not basing your claim on a representative sample of cases (to be representative you’d need to also sample days from November, February, etc.).

Know, though, that you can make a valid generalization, which must meet two criteria:

1. A valid generalization must be made on the basis of a sufficient number of cases.
2. The cases must compose a representative sample of ALL cases.

Transfer Fallacy

Transfer fallacies extend reasoning beyond what is logically possible. Three types exist:

1. Fallacies of composition occur when claims assert that what is true of a part is true of the whole. For example “,Voters in Austin tend to be Democrats; therefore, voters in other counties in Texas must be Democratic as well,” or “When you’ve seen one zoo, you’ve seen them all.”

2. Fallacies of division occur when claims assert that what is true of the whole is also true of its parts. For example, “Communication courses are fun, and Public Speaking is a communication course; therefore, Public Speaking is fun.” (We hope it is fun, but stated this way, the conclusiothat Public Speaking is fun is a logical fallacy.)

3. Fallacies of refutation, also known as the Straw Man Argument, occur when an arguer attempts to draw attention to the successful refutation of an argument that was never raised or restates a strong argument in a way that makes it appear weaker.  Such arguments are forms of deception because they introduce bogus claims, claims that were not part of the original argument, or they misrepresent the original claim. For example, “Your argument that drunk driving causes death and injury is very interesting, but what about all the people who weren’t wearing seat belts at the time of the accident? Aren’t you assuming every person involved in an automobile accident has been drinking?” You can’t really make that claim until you look at additional information.

Irrelevant Arguments

Also known as non sequiturs (Latin for “it does not necessarily follow”), irrelevant arguments are ones that do not seem pertinent in terms of the claims they advance on the basis of proof they offer. They make assumptions that do not follow from the information provided. For example, “Rolling Valley Vineyards must make great wines (claim). Their social media posts state that they are the only winery that doesn’t use pesticides to control insect damages to their vineyards. We should all be concerned about pesticides in what we eat and drink” (the proof does not follow from the claim).

Circular Reasoning

Also known as begging the question, arguments that are circular support claims with reasons identical to the claims themselves. For example, “Everyone at school loves Michele because she is so popular. ”

Avoiding the Issue

Avoiding the issue is an error of reasoning that shifts attention from the issue under consideration. It can take takes the form of:

• a simple evasion of the issue: changing the subject for no apparent reason

• an attack on the arguer rather than the argument (an ad hominem attack)

• a shift of ground: when an arguer abandons their original position and adopts a new one

• seizing on a trivial point rather than the central issue: when you magnify the weakest or most indefensible position of the other side all out of proportion to its actual importance

Either/Or

A forced either/or situation is one in which audiences are presented with an oversimplified either-or choice without including other logical choices, phrased in such a way that it forces them to favor the arguer’s preferred option. For example, “We must censor video games because to not do so is to support the indefensible notion that video games do no damage to society when we know full well that they have major detrimental effects.”

Appeals to Ignorance

Known as ad ignoratium in Latin, appeals to ignorance ask the audience to accept the truth of a claim because no proof to the contrary exists. In other words, something is true because it cannot be proven false. For example, “The inability to disprove the existence of flying saucers and extraterrestrial visitation to earth confirms the existence of the former and the occurrence of the latter.”

Appeal to the People

Am appeal to the people, also known “bandwagon” or, in Latin, the ad populum argument, occurs when a claim is justified based only on its popularity. That is, it assumes that you should believe the claim because the majority of people believe the claim. For example, “Everyone is getting stars tattooed on their belly, so stars must be great and you should get one too!,” or “Public opinion polls show that the majority of Americans favor congressional term limits; therefore, we ought to pass a constitutional amendment that limits the terms of members of Congress.”

Appeal to Authority

While an argument that uses the opinions and testimony of experts can be a legitimate form of reasoning, there are cases where such reasoning can be used carelessly and incorrectly. Appeals to authority become logical fallacies when they 1) appeal to a seemingly authoritative source that lacks real expertise. Abuses of authority commonly involve texts such as the Bible, the Constitution, revered persons, or testimonials by celebrities in advertising. For example, “I’m not a doctor but I play one on TV; therefore, you should use the brand of aspirin that I’m advertising.”

Appeal to Tradition

An appeal to tradition asks an audience to accept something just because it is customary rather than because there are reasons that justify it. For example, “The Democratic party has always been the party of the environmental movement. Therefore it makes no sense for the Sierra Club to endorse a candidate who is not a Democrat.”

Note that dealing with logic may seem overwhelming when you create support for persuasive speech.  To deal with logic, it may help to put yourself in a sceptic’s position.  If you’re someone in the audience who is not prone to accept what you’re saying, what questions might you have?  What points of evidence would you contest?  Try creating your speech content first, and then editing for logic after you create your draft.