Eight Types of Evidence – Strengths and Weaknesses

Overview: The ability to distinguish sources of evidence allows students to better evaluate and generate information in support of arguments.

Evidence is a huge component of reasoning and argument. Understanding how evidence works and how it might be questioned, probed, or attacked, significantly boosts students’ reasoning ability. The following material offers a vocabulary that can operate as a toolkit for use on any task that requires analysis or generation of evidence.

1. Personal Experience – It happened to you. You know what bronchitis feels like because you had it last year, and it was terrible.

Strengths – Emotionally intense and relevant, collected by your very own senses.

Weaknesses – The way you interpret your own experiences is very personal and based on your own expectations and biases. Also, your senses have all sorts of flaws, as does your memory. You remember events and moments that are bizarre, intense, or otherwise of interest to you, which is a small sliver of the world around you.

2. Personal Observation – You saw or measured the event. You haven’t had a migraine, but your mom gets them and you have witnessed how painful and awful they can be.

Strengths – Collected by the senses, scientific measurement techniques can carefully and cleverly isolate the information you are seeking.

Weaknesses – The same as Personal Experience, scientific measurements can be corrupted by factors you didn’t anticipate.

3. Testimonial – The experience or observation of someone else; a witness. My friend saw a guy with pink eye yesterday. He said it was pretty gross.

Strengths – They were there, emotional weight of hearing someone’s story or claim. We want to believe one another because lying is so dangerous to our social fabric.

Weaknesses – The person might be mistaken (see weaknesses of Personal Experience), lying, or leaving out important details.

4. Appeal to Authority – The experience or observations of a learned and/or respected person; an expert. My brother is a doctor and treated a guy with a broken arm. He told me that broken bones don’t always hurt as much as you would expect.

Strengths – This person presumably has a lot of access to information, a depth of experience, and a professional reputation on the line.

Weaknesses – Same as Testimonial, the person’s expertise could be based on a depth of experience in field separate from the one we’re dealing with. (See Appeal to Questionable Authority Fallacy).

5. Case Examples – Historical, literary, or other recorded examples. They could be the statements of witnesses or experts, or they could be more general events that we cite to support our claim. War is terrible for soldiers on the ground. You can read all about it in many Civil War diaries.

Strengths – Same data available to everyone, you can carefully seek out and find examples that support your claim (see Confirmation Bias), emotional weight of vivid examples.

Weaknesses – Examples might be isolated and/or unrepresentative of “normal” experience (see Hasty Generalization).

6. Research Studies – Large sample of carefully gathered  information scrutinized with statistical tools and peer-reviewed by other experts.

Strengths – Large samples protect against Hasty Generalizations, the same data is available to everyone.

Weaknesses – There is a long list of potential pitfalls to good research. They include poor design, poor data gathering, and poor data analysis. There are conflicting studies which cite different parts of the same data, and there are weak studies published to push a political agenda.

7. Analogy – Citing a similar circumstance; if it worked in that ugly situation, it will work in this ugly situation. If cigarettes give mice cancer, they probably give humans cancer.

Strengths – Much of life follows general rules; if something works in one place, there’s a pretty good chance it will work in another place.

Weaknesses – Places can be different! You have to look at salient details (a.k.a the details that actually contribute to whatever it is you are looking at). If a flying squirrel can fall from a tall building and survive, I should be able to do the same thing. We are both mammals! (See Bad Analogy Fallacy.)

8. IntuitionYour gut feeling, presumably based on years of experience. It feels true. The inferences that pop into your head first are likely to be based on intuition rather than research studies or other types of evidence. If you hear a bump in the night, the weight of your experience will offer a causal inference, and if that inference isn’t dangerous (“it was just the wind!”), you will likely just go back to sleep.

Strengths – For most issues, our experience is a good guide to life. We have built a pretty good picture of the world, and we can generally rely on it to stay consistent. Malcolm Gladwell explains the power of quick inferences in his book Blink, and Daniel Kahneman describes it as “fast thinking” in his book Thinking, Fast and Slow.

Weaknesses – Your experience is personal and unique. Other people have had different experiences and will therefore have different gut feelings. There is no way to prove that your intuition is correct. If people trust it, it’s because you have been right many times in the past and will therefore trust you to be right again (see Appeal to Questionable Authority). All of the fallacies and biases that lead us to make weak inferences are relevant here.

Again, this list is adapted from Asking the Right Questions by Neil Browne and Stuart Keeley, which offers a more in-depth look at each type of evidence. I’ve simplified and adapted their work to serve as an introduction to students new to this approach.

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