Fallacies are errors in reasoning that can damage a persuasive speech. Evidence can be defective when the speaker misuses facts, statistics, and testimony. Common errors include the slippery slope fallacy, which assumes that a single instance will establish a trend, the confusion of fact with opinion, and the red herring fallacy, using irrelevant material to divert attention from the issue. Statistical fallacies include the myth of the mean, in which averages create illusions that hide reality, and faulty conclusions based upon flawed statistical comparisons. Evidence can also be used inappropriately, featuring facts and figures when the situation calls for examples, examples when the audience needs facts and figures, testimony to hide the weakness of information, or narratives to justify unethical behavior.
Various defects can reduce the value of proof. Speakers misuse proof by ethos when they commit an ad hominem fallacy, attacking the person rather than the argument. When speakers merely assert and assume in their conclusion what they have not proved, they commit the fallacy of begging the question.
Fallacies are also common in the patterns of argument. If your major premise is faulty, the entire argument will crumble. Other frequent errors occur when probability is passed off as certainty, and when the speaker confuses association with causation, reasoning that if something happened after an event, it therefore was caused by the event. This is called the post hock fallacy. A non sequester fallacy occurs when irrelevant conclusions or evidence are introduced into argument. Inductive reasoning can suffer from a hasty generalization drawn from insufficient or non representative observations. Argument by analogy is defective when important dissimilarities outweigh similarities.
Either—or thinking can be a special problem in speeches calling for action. This fallacy reduces audience options to only two, one advocated by the speaker, the other undesirable. When speeches that contend with opposition understate, distort, or misrepresent an opposing position for the sake of easy refutation, they commit the straw man fallacy.