Evidence

Contents

• Evidence in the Context of Argument
• Locating Credibility of Evidence
• Tests of Evidence
  • General Tests
  • Tests of Stories
  • Tests of Statistics
  • Tests of Visuals
  • Tests of Analogy
  • Tests of Authority
• Applying the tests of argument
• Exercise on Evidence

Evidence in the context of argument

Evidence is information or explanation provided in support of an argument. The rhetor selects evidence that s/he believes will warrant the claim of the argument. Thus, different selections of how to warrant an argument lead to the selection of different tests to challenge argument.

Locating Credibility of Evidence

When a rhetor offers evidence to support a claim, where does s/he base belief that your faith in the claim will be enhanced by the evidence:

• Primary Evidence: The speaker reports as an observer or as an expert. Credibility requires assessing the rhetor as the source. Note the link to ethos.
• Secondary Evidence: The speaker relies on others, often experts. Credibility requires assessing the named source. Note this is argument by authority.
• Enthymatic: The speaker relies on the audience's experiences or expertise. Credibility requires the audience's having had the experiences or knowing the claim to be true. Note the connection to audience analysis.
• Direct: The audience can see or hear the evidence for themselves. Note this is visual evidence.

Tests of Evidence

General Tests

• Internal Consistency: Is there a contradiction within the evidence? Contradictions diminish faith in the claim.
• External Validity: Does the evidence ring true with your experience? We compare claims with our own experience. If the claim is consistent with our experience we have more faith in it. If it is not, we demand better evidence to offset our experience.
• Corroboration: Is the claim supported by other evidence? The more evidence we have that supports a claim, the more faith we have in it.
• Bias: Is there a reason for the source of the evidence to shade the truth? We examine the motives of people providing evidence to understand if they have an interest in deceiving us. If they have an interest different than ours, we have less faith in them as sources of evidence.

Tests for Stories

See Campbell and Huxman

Stories are narratives with characters and plot development. They have an impact on clarity and understanding as well as warranting claims.

• Is the story typical? If we suspect the story may be unusual, we have less faith in it for supporting a generalized claim.
• Is the story coherent and consistent? Stories that do not hang together, seem incomplete, do not quite make sense diminish our faith in the claim. Internal validity above.
• Does the story ring true with your experiences? External validity above.

Tests for Statistics

See Campbell and Huxman

Statistics are usually systematic reports of statistical study and thus dependent on authority tests. Note that statistics Only prove scope or frequency of problem. There are specific tests for statistics in addition to the tests you apply if the statistics come from an authority.

• Have the statistics been collected carefully?
• Are the quantified concepts defined carefully? Any statistic measures a concept. For example, an opinion poll asking "Do you support the President?" turns on the understanding of the term support. Is the concept one that is precise enough that the statistic makes sense.
• Are the questions phrased clearly and fairly? Surveys are only as good as the quality of their questions. Polls may be "push polls" or polls that influence their own responses. Example: When do you think the President first became concerned about the economy? assumes the President once was not.
• Has the sample surveyed been drawn fairly? In sampling, a statistic describes a small number of individuals drawn from the relevant population, describes their responses and generalizes to everyone in that group. To be a good sample, every individual in the population should have had an equal chance of being in the sample. Otherwise, the statistic is suspect.
• Does the statistic reported make sense? Statistics must be presented in such a way that they make sense to you.

Tests for Visuals

See Campbell and Huxman

Visuals enter our consciousness through direct sight rather than through words. They include graphs, pictographs, photographs, or drawings. Note that visuals may support example, sign, or analogy.

• If you are evaluating an image, see the tests for stories or analogies, whichever applies.
• If you are evaluating a graph depicting statistical claims, see tests of statistics.

Tests for Analogy

See Campbell and Huxman

Analogies provide evidence of a similar instance seeking to transfer your knowledge of the similar instance to the instance at hand.

• What are the similarities between instances?
• What are the differences between instances?
• Apply tests of stories to assess accuracy of the description of the analogical instance
• How do similarities and differences affect the power of evidence to warrant the claim?

Tests for Authority

See Campbell and Huxman

Authorities may be witnesses -- testimony given because they were in a position to observe -- or expert -- testimony given because they have studied the thing they testify about.

• Was the source in a position to observe or know?
• Is there self-interest or other reason the source might shade the testimony?
• Is the expert within his/her area of expertise?

Applying the Tests of Argument

To apply the tests that you have learned to evaluating argument:

1. Identify the claim
2. Identify the evidence supporting the claim
3. Apply the general tests of evidence
4. Classify the evidence according to one of the types of argument you have learned
5. Apply the tests appropriate to the type
6. Write your evaluation based on your answers