Welcome or Title

Required Reading

Logic & Contemporary Rhetoric Appendix (359-367)
Logic & Contemporary Rhetoric Chapter 10
The 4 Types of Evidence
Evaluating Your Evidence

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Types of Evidence
Summary of the Rules of Evidence

Introduction

"We better know there is fire whence we see much smoke rising than we could by one or two witnesses swearing to it. The witnesses may commit perjury, but the smoke cannot." ~ Abraham Lincoln

Lincoln was alluding to the qualitative difference between concrete evidence and hearsay. This week we will be talking about categories of evidence as well as types of evidence.

5a Evidence 1 - Categories

There are myriad ways of sorting through evidence. We will arbitrarily use categories and types as the terms, with categories representing the broad groups into which different specific types of evidence might be sorted. The three categories we will use are primary, secondary, and tertiary -- or 1, 2, and 3.

 

 

5b Evidence 2 - Types

This discussion will focus on what I will call the types of evidence that are used in most academic essays.

There's a good explanation of these at Texas A&M. I've copied that explanation for you:

Types of Evidence

The goal of an argumentative essay is to persuade readers by providing evidence to justify reasons supporting your thesis. To strongly support each reason, writers should use different types of evidence containing specific, verifiable details. Here are five common examples of evidence:

• An anecdote relates a brief story; it explains what happened, who was involved, and where and when it happened.
  • Sample reason:  Students commuting to Texas A&M International University should carpool because it makes driving to and from the university safer.
  • Anecdotal Evidence: Joe, Carla, Anna, and I live in Zapata. We attend TAMIU and carpool to school three days a week. During the hour-long drive, we discuss our coursework, complain about our jobs, and share opinions on movies we’ve seen. Our chat sessions help pass the time and keep Joe from falling asleep behind the wheel, especially on the long, dark drive back to Zapata.
• Statistics support a reason by using specific numerical data such as percentages, fractions, decimals, whole numbers, and/or ratios. The writer must report statistics accurately and objectively.
  • Sample reason:  Students should carpool to campus because it saves money.
  • Statistical Evidence: Last semester I spent approximately $22.50 on gasoline each week. Since I’ve started carpooling to class this semester, I spend 40% less on gasoline, a savings of $9.00 per week and $135 per semester.
• An authority or expert is a person or an organization with credentials, such as educational degrees, publication credits, work experience, professional presentations, etc., that demonstrate his or her expertise on the topic discussed.
  • Sample reason:  Carpooling reduces the risk of traffic accidents.
  • Authoritative Evidence: According to Officer Jose Montoya, an eighteen-year police veteran, carpooling reduces the number of cars on the road, which means less traffic congestion and, therefore, fewer accidents.
• An analogy is a comparison between two distinctly different things that still have at least one significant, relevant similarity.
  • Sample reason:  Carpooling reduces air pollution.
  • Analogy as Evidence: A room where only two people smoke is significantly less polluted than a room of the same size with more than two people smoking. Similarly, our city will have less air pollution if we carpool because fewer cars produce fewer pollutants.
• A hypothetical situation is a believable but fictional incident with enough detail to feel real. A writer using a hypothetical situation should create details that tell what happened, to whom it happened, and where and when it happened. However, hypothetical situations are considerably weaker evidence than the other methods because they’re not factual.
  • Sample reason:  Students who carpool to class have more time to study.
  • Hypothetical Situation as Evidence: Driving to and from the university takes up study time. Suppose that Alex, biology major, who drives to campus three nights a week, spends about thirty minutes driving to class each night for a total of ninety minutes a week. He could carpool with two students and drive to school on one night rather than three. His driving time shrinks to thirty minutes a week, giving him a full study hour on the way to class.

Where would you place the examples from the categories of evidence graphic? Exactly. Those examples could be used in a variety of types of evidence. A letter might be considered anecdotal or authoritative depending on its author and its content. In addition to identifying evidence, we'll need to talk about how it is used and which types are the strongest.

Take a look at this video. The author is making an argument. What sorts of evidence are being used? Which ones might be considered strong? Which ones might be considered weak? Why?

Is he right? I haven't got a clue, though I suspect there are a number of flaws in his reasoning, so I'm not persuaded by his argument. But I do know that most of the scientific community would be hard pressed to even consider Adams' theory because of their own observational bias. So, you may find yourself up against FACTS and DATA you didn't even know existed. If that's the case, you will need to find some way to determine whether or not the facts should be accepted, and if they should, whether or not they should have an effect on your thinking.

Consider why empirical evidence is usually considered quite strong.

 

Empirical or scientific research results in primary or direct evidence. These are facts that can stand alone. For example, DNA is direct evidence. The existence of DNA has been proven scientifically.

 

In the empirical cycle described by A.D. de Groot, each step in the process contributes to the eventual conclusion. The process includes:

• Observation (the gathering of data)
• Induction (formulation of the hypothesis)
• Deduction (consequences of hypothesis as testable predictions)
• Testing (testing of the hypothesis)
• Evaluation (analyzing outcome of testing)

The empirical cycle is part of the scientific method of research. What makes the scientific method particularly valuable as evidence is that its conclusions are reproducible. Once testing and evaluation have been done, another may choose to reexamine the cycle to determine whether it holds. The more times a cycle is examined and holds true, the stronger the implication of the conclusion. And they are often physical; they can be seen, touched, heard, smelled, or even tasted (if one is brave enough). Scientific conclusions can stand up to scrutiny.

 

Circumstantial evidence requires inference -- a sort of educated guess -- to reach a conclusion. Take this flat tax argument as an example:

• Taxes would be set at a single flat rate.
• There would be no bias against savings & investment.
• Flat tax would end discrimination in taxation.
• A flat tax would simplify the taxation process.
• A flat tax would stimulate economic growth.
• A flat tax would result in higher incomes.
• A flat tax would result in more jobs.
•    THEREFORE…
• The nation should switch to a flat tax.

Notice in particular that no concrete evidence has been provided. Instead, a series of claims has been made. The arguer's hope is that the number of points would be convincing and cause the recipient to agree with the arguer's conclusion. This sort of arguing through a process of reasoning is often effective. But caution must be used when considering a circumstantial argument. In the flat tax example, there are assumptions, twisting of facts, and unsupportable claims. If a recipient accepts the chain of reasoning without further investigation, it's likely a poor decision would be made.