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Unlike deduction, which reaches a conclusion based on information provided by the major and minor premises, induction uses what you know to make a statement about something that you don’t know. While deductive arguments can be judged in absolute terms (they are either valid or invalid), inductive arguments are judged in relative terms (they are either strong or weak).

You reach an inductive conclusion by making an inference—a statement about what is unknown based on what is known. (In other words, you look at the evidence and try to figure out what is going on.) For this reason, there is always a gap between your observations and your conclusion. To bridge this gap, you have to make an inductive leap—a stretch of the imagination that enables you to draw an acceptable conclusion. Therefore, inductive conclusions are never certain (as deductive conclusions are) but only probable. The more evidence you provide, the stronger and more probable your conclusions (and your argument) are.

Public-opinion polls illustrate how inferences are used to reach inductive conclusions. Politicians and news organizations routinely use public-opinion polls to assess support (or lack of support) for a particular policy, proposal, or political candidate. After surveying a sample population—registered voters, for example—pollsters reach conclusions based on their responses. In other words, by asking questions and studying the responses of a sample group of people, pollsters make inferences about the larger group—for example, which political candidate is ahead and by how much. How solid these inferences are depends to a great extent on the sample populations the pollsters survey. In an election, for example, a poll of randomly chosen individuals will be less accurate than a poll of registered voters or likely voters. In addition, other factors (such as the size of the sample and the way questions are worded) can determine the relative strength of the inductive conclusion.

As with all inferences, a gap exists between a poll’s data—the responses to the questions—and the conclusion. The larger and more representative the sample, the smaller the inductive leap necessary to reach a conclusion and the more accurate the poll. If the gap between the data and the conclusion is too big, however, the pollsters will be accused of making a hasty generalization (see Hasty or Sweeping Generalization). Remember, no matter how much support you present, an inductive conclusion is only probable, never certain. The best you can do is present a convincing case and hope that your audience will accept it.