Evaluating Evidence Help

Updated on Sep 28, 2011

Introduction to Evaluating Evidence


"You cannot lie ever, because a lie destroys the credibility of the product, and credibility is more important than anything. Credibility's even more important than clarity."

—Frank Luntz, American political consultant (1962–)

Lesson Summary

Now that you're able to separate the conclusion from the premises that support it, it's time to evaluate those premises. This is a vital step; the conclusion, after all, is trying to convince you of something—that you should accept a certain opinion, change your beliefs, or take a specific action. Before you accept that conclusion, therefore, you need to examine the validity of the evidence for that conclusion. Specifically, there are three questions to ask yourself when evaluating evidence:

  1. What type of evidence is offered?
  2. Is that evidence credible?
  3. Is that evidence reasonable?

Types of Evidence

There are many different types of evidence that can be offered in support of a conclusion. One of the most basic distinctions to make is between premises that are fact, premises that are opinion, and premises that can be accepted only as tentative truths.

Before going any further, here's a review of the differences:

  • A fact is something known for certain to have happened, to be true, or to exist.
  • An opinion is something believed to have happened, to be true, or to exist.
  • A tentative truth is a claim that may be a fact but that needs to be verified.

Whether they're facts, opinions, or tentative truths, premises can come in the following forms:

  • Statistics or figures
  • Physical evidence (artifacts)
  • Things seen, felt, or heard (observations)
  • Statements from experts and expert witnesses
  • Reports of experiences
  • Ideas, feelings, or beliefs

Of course, some types of evidence seem more convincing than others. That is, people are often more likely to believe or be convinced by statistics than by someone's opinion. But that doesn't mean that all statistics should automatically be accepted and that all opinions should be rejected. Because statistics can be manipulated and because opinions can be quite reasonable, all forms of evidence need to be examined for both credibility and reasonableness.

For example, the reasonableness of statistics can't really be questioned, but their credibility must be questioned. Similarly, any feeling or belief should be examined for both credibility and reasonableness.

Credible Evidence

Whatever the type of evidence the arguer offers, the first thing that needs to be considered is the credibility of the arguer. Is the person making the argument credible? Second, if the arguer offers evidence from other sources, the credibility of those sources needs to be questioned. If both the arguer and his or her sources are credible, then the argument can tentatively be accepted. If not, the argument shouldn't be accepted until it is examined further.

First, here's a review of the criteria that determine credibility. To be credible, a source must:

  • Be free of bias
  • Have expertise

Expertise is determined by:

  • Education
  • Experience
  • Job or position
  • Reputation
  • Achievements

In the case of an eyewitness account, the following must be considered:

  • The witness's potential for bias
  • The environment
  • The physical and mental condition of the witness
  • The time between the event and recollection of the event


Do you remember covering primary and secondary sources in history class? Primary sources are the ones you can trust the most for accuracy because they are/were written by whom? By eyewitnesses! They were created by people who were really there at the moment rather than read about it, interviewed someone about it or other secondary sources.

Here is a short deductive argument. Read the following passage carefully*:

Current statistics show that 15% of children are obese. Childhood obesity increases the risk for developing high blood pressure, Type 2 diabetes, and coronary heart disease. In fact, 80% of children diagnosed with Type 2 diabetes are overweight. Being obese also lowers children's self-esteem and affects their relationships with their peers. This growing epidemic can be attributed to several factors: genetics, lack of physical activity—children are spending more and more time in front of the television and the computer—and lack of nutritional education. If children were educated about nutrition and exercise, then obesity rates would decline significantly. That's why we must pass a law that requires that nutrition and exercise education be part of the school curriculum for all students in grades K–12. Unfortunately, it's too late for my 12-year-old brother; he's already been diagnosed with Type 2 diabetes. But we must take measures to improve the health and well-being of future generations to come.

*The statistics in this text are fictitious and meant to serve purely as examples.

First, identify the conclusion in this passage. What is the overall claim or point that the passage is trying to prove? Once you identify the conclusion, underline it.

You should have underlined the claim, "We must pass a law that requires that nutrition and exercise education be part of the school curriculum for all students in grades K–12." The phrase "That's why" may have helped you identify this idea as the main claim. (If you had trouble, take a moment to review Lesson 7, "Working with Arguments.") The following table lists the premises that support this conclusion. Note that not every sentence in this argument is a premise.

The arguer's experience offers an important clue here about her credibility. Because of what happened to her brother, is she likely to be biased on the issue? Absolutely. However, does this rule her out as a credible arguer? Not necessarily. Chances are that if her brother was diagnosed with diabetes due to poor nutritional habits, she knows more about the issue than the average person. In other words, her experience indicates that she has some level of expertise in the area. Thus, though there's evidence of some bias, there's also evidence of some expertise. Because there is both bias and expertise, the argument needs to be examined further before you can determine whether or not to accept it.

Is the arguer's experience credible? Well, it can be assumed that she's telling the truth about her brother being diagnosed. Is her opinion credible? That depends on her own credibility, which is still in question, and the reasonableness of that opinion, which is covered in the next section.

The next step is to consider the credibility of premises provided by the outside source; that is, the statistics offered about childhood obesity. Notice that here the arguer doesn't give a source for the figures that she provides. This should automatically raise a red flag. Because numbers can so easily be manipulated, it's crucial to know the source of any figures offered in support of an argument.

Evaluating Evidence

View Full Article
Add your own comment