Evidence, or “Why should I believe it?”

Disclaimer: This is currently a “working draft.” Grammar correction, additional content, formatting and media still need to be added before this is considered complete.



This is the first of a 5 part series I’m titling “Evidence or “Why should I believe it?” It’s an essay I’ve been wanting to write for almost a year now and is to be included in an future critical thinking course that was inspired by several sources (including but not limited to: an excellent college level critical thinking textbook that I highly recommend: Asking the Right Questions, A Guide to Critical Thinking, 9ed (however, the 10th ed is out now), the lectures of 2 of my university professors, the input and guidance of another professor, and my own personal knowledge accumulated through my minor in philosophy).

The first part of the series describes what evidence is and its property of quality. In addition I’ll go over 5 of 9 major types of evidence after which the more popular types (and thus, more lengthy exploration) will follow in their own threads. My plan is to have the next in the series go live 1-3 days after the previous part. I decided to break this essay into parts instead of have it all in one thread because I felt it would be easier to digest and thus allow for more efficient and relevant discussion.

The reason for this essay is to give readers an stronger tool in their critical thinking “toolbox” that will hopefully create stronger arguments. Often times we think that as long as we have a response to the question “Do you have any support?“, that it is sufficient to support our claims and do so in an adequate way. But as explained throughout this essay, this is simply not the case. There are a variety of types of evidence that we must consider and each has its certain quality or dependability. We ought to strive to use the most dependable evidence we can in our arguments. It is my hope that this essay (and its following sections) will help make those arguments stronger and our participants just a little bit wiser.

I do not claim to be an expert. I do not claim to not need to grow more in my ability to evaluate arguments and evidence. I do not claim to have the answer to every challenge in regard to proper argument analysis. And I do not claim to be able to consistent 100% of the time in providing proper form and proper evidence in my arguments. It is a practice that we all must continue to practice and strengthen with never ceasing focus and energy. I do claim to have a fair grasp of argument analysis having studied fairly extensively both formally and informally in the fields of critical thinking and philosophy. I’ve also worked as a TA for a professor in a critical thinking course and have almost completed my minor in Philosophy while maintaining a 4.0 GPA, making the Dean’s List.

Lastly, there may be a few typos and errors. As I am made aware of them, I will correct accordingly. If you have any suggestions or ideas on how to further expand on the ideas presented in this essay, please share them and I will consider revising this essay as needed. For those who are curious, I created the flowcharts using iMindmapInspiration, and Snaggit.

The Outline

The following is the outline for this essay, which is of course followed by the corresponding and detailed text.

I Evidence is the Reason to Believe a Factual Claim.
II Dependability of Evidence

1. Evidence defined in more detail

III The Types of Evidence

1. intuition
2. personal experiences or anecdotes
3. testimonials
4. personal observations
5. case examples
6. appeals to authorities or experts (Part 2)
7. research studies (Part 3)
8. analogies (Part 4)
9. statistics (Part 5)


The use of the italic green style denotes that the term has a particular definition as it is to be understood in this essay. The definition can be found below the section that the term is first used in. This is for convenience purposes only.

I. Evidence is the reason to believe a factual claim.

Generally speaking, evidence is the answer to the question “Why should I believe it?” Any time someone makes a factual claim ) this question ought to be asked. Ideally, the answer (evidence) is provided with the factual claim but often times it just isn’t. There are many reasons that evidence isn’t immediately offered when making a claim. Some reasons could be:

  • the claim maker assumes the reasons are known to their audience
  • the claim maker forgot to offer the reason(s)
  • the claim maker isn’t quite sure why they believe and thus could not provide a compelling enough reason
  • the claim maker doesn’t believe the reason for the claim is without its challenges and hopes that the claim can stand on its own without much scrutiny
  • the claim maker is being deliberately deceiving with the claim and by offering reasons, the bad claim would be outed

Whatever the reasons for not providing the evidence to the factual claim, the fact remains that the audience still needs to know why they should believe the claim. And if the reasons are not offered up front, we should be expected to answer the question “Why should I believe it?” when asked by our audience.

When a claim is made and no evidence is offered, it is merely an assertion (as opposed to an argument). And as members of the audience we should never just accept the claim maker’s assertion. Assertions are not dependable! Nor are they compelling. An assertion then has no value when it comes to persuade others to what is perceived by the claim maker’s truth. Assertions ought to be avoided in debates here at ODN (and elsewhere if the goal is to persuade or convince). And that is what a claim without evidence (a reason) is…a mere assertion.

So, if you are the claim maker and have not offered a reason to believe the claim, you have only offered an assertion. Try to avoid this and instead offer at least one reason for your audience to believe the claim up front. If you are the audience evaluating the claim then you ought to be asking either yourself or the claim maker why the claim ought to be believed. It is a good idea to ask yourself first perhaps, because the reason may actually exist in the argument. Some arguments can be rather complex and the reasons for the conclusion may not be easily identifiable. I’ll cover that in another topic though.

Lastly, there is a difference between the issues of “Why should I believe it?” and “Is the reason persuasive enough for me to believe it?” The former is merely addressing the issue of reasons existing. The latter is addressing the quality of the existing reasons. A bad quality reason is not likely to be persuasive whereas a reason that is credible, well thought out, of high quality, will be more likely to be persuasive.

factual claim

beliefs held and/or statements made about the way the world was, is or will be, AND is what the communicator wants us to accept as “fact”; factual claims can be conclusions, reasons, or even assumptions (descriptive assumptions and value assumptions will be covered in a future topic)

assertion - a claim without stated evidence
evidence - generally speaking: reasons that support the claim, however see below for a more formal and detailed definition
argument - a conclusion supported by stated reasons
reasoning - used interchangeably with argument; additionally both could be thought of as “the use of one or more ideas to support another idea

II. How good is the evidence?

The next question we have to ask (after we have identified the reasons to believe the claim) is “How good is the evidence?” or “How good is the reason for believing the claim?

To evaluating reasoning (or the argument – which is the conclusion supported by stated reasons/evidence) we need to remember that some factual claims can be counted on more than others. For example, you’d probably feel quite certain that the claim “most US senators are men” is true, but less certain that the assertion “practicing yoga reduces the risk of cancer” is true. Which brings us to another important issue…dependability.

Because it is extremely difficult, if not impossible, to establish the absolute truth or falsity of most claims, rather than ask whether they are true, we ought to ask whether they are dependable. In essence, it’s asking “Can we count on such beliefs?” The greater the quality and quantity of evidence supporting a claim, the more we can depend on it, and the more we can call the claim a “fact.”

For an example, abundant evidence exists that George Washington was the first President of the United States of America. Thus, we can treat that claim as a fact. On the other hand, there is much conflicting evidence for the belief “alcoholism is a disease.” We thus can’t treat this belief as a fact. The major difference between claims that are opinions and those that are facts is the present state of the relevant evidence. The more supporting evidence there is fora belief, the more “factual” the belief becomes.

- the quality of the evidence; the greater the quality the more we can call the claim a “fact”

II a. So what is evidence exactly?

I said above that “evidence is the reason that supports the claim”, and while it is true, it is still too general and needs further explanation. It doesn’t adequately answer the question “When should we accept a factual claim as dependable?” Quality of evidence varies and for a factual claim (belief about the world was, is, or is going to be that the communicator wants us to accept as factual) to be considered dependable (or factually accurate) there must be a way to measure the given evidence (reasons to believe). There are three instances in which we will be most inclined to agree with a factual claim:

  1. When the claim appears to be undisputed common knowledge, such as the claim “weight lifting increases muscular body mass;”
  2. when the claim is the conclusion from a well-reasoned argument, and
  3. when the claim is adequately supported by solid evidence in the same communication or by other evidence that we know.

#1 and #2 are givens, so our concern here is with the 3rd instance. Determining the adequacy of evidence requires us to ask, “How good is the evidence?” To answer this question, we must first ask “What do we mean by evidence?” Here is our formal, detailed information:

 - explicit information shared by the communicator that is used to back up or to justify the dependability of a factual claim. In prescriptive arguments (“ought” or “should” arguments; moral arguments) evidence will be needed to support reasons that are factual claims; in descriptive arguments, evidence will e needed to directly support a descriptive conclusion.

Here is a “Factual Claim Evaluation” flowchart I’ve created for ease of understanding the process:

Now that we have a working definition of evidence and an example of how we can ask certain questions to move forward with our evaluation, we can now discuss the quality of evidence in more detail. The quality of evidence depends on what kind of evidence it is. Thus, to evaluate evidence, we first need to ask “What kind of evidence is it?” Knowing the type of evidence will be needed to directly support a descriptive conclusion. Not all types, as we will see, have the same quality. Some are strong, some are very weak (and ought to be avoided).

———- Post added at 10:07 PM ———- Previous post was at 10:06 PM ———-

III. What are the types of evidences?

The major types of evidence include:

1. intuition
2. personal experiences or anecdotes
3. testimonials
4. personal observations
5. case examples
6. appeals to authorities or experts (Part 2)
7. research studies (Part 3)
8. analogies (Part 4)
9. statistics (Part 5)

When used appropriately, almost every type of evidence can be “good evidence.” But we have to keep in mind that there is no evidence that will be a slam dunk that gets the job done conclusively. We are looking for better evidence, not just a single, smoking gun that settles the issue once and for all. And depending upon what is being claimed a certain type of evidence will have more weight than another (thus used appropriately). Let’s take a look at these types in more detail.

1. Intuition

Intuition refers to a process in which we believe we have direct insights about something without being able to consciously express our reasons. It’s a gut feeling, belief in “direct insight” without ability to express reasons.

Example: I just have this feeling that Senator Goodall will surprise the pollsters and win the election.

When we use intuition to support a claim, we rely on “common sense,” or on our “gut feelings,” or on hunches. When a communicator tells us that something is true because “common sense tells us” or “I just know it it is true,”, they are using intuition as their evidence. But a major problem with using intuition as evidence is that it is private; others have no way to judge its dependability. Thus, when intuitive beliefs differ, as they often do, we have no solid basis for deciding which ones to believe. Also, much intution relies on unconscious processing that largely ignores relevant evidence and reflects strong biases. Consequently, we must be wary of claims backed up only by intuition.

However, sometimes “intuition” may in fact be relying on some other kind of evidence, such as extensive relevant personal experiences and readings, that have been unconscious accessed from somewhere in our mind. For example, when an experienced pilot has an intuition that the plane doesn’t feel right as it taxis for takeoff, we might be quite supportive of further safety checks of the plane prior to takeoff. Sometimes “hunches” are not blind, just incapable of explanation. As critical thinkers, we would want to find out whether claims relying on intuition have any other kinds of evidential support.

Generally speaking, the quality of evidence that intuition offers is not very good in terms of a debate. It’s more useful in situations in which relevant personal experience comes into play (pilot in our scenario above or a police officer witnessing suspicious behavior) that is usually only obtained through quite a bit of training and routine experience.

Problem: Private – can’t be evaluated by others, no way to distinguish between conflicting intuitions, often reflects bias. Generally unreliable, very weak form of evidence.

 When the intuition belongs to someone with unique experience in the area or with the event or object the claim concerns, becomes better evidence – provides a reason to dig deeper.

Here’s an “Evaluation of intuition (as evidence)” flowchart:

2. Personal Experiences and Anecdotes

Personal experience refers to the individual observances and experiences we each have (or the personal experience of someone known to the speaker); it only address the possibility of something, not the probability of it.

Example: “My friend Bob does really well on his tests when he stays up all night to study for them; so I don’t see the need for getting sleep before taking tomorrow’s test.”

Arguments such as “Chocolate makes me feel better so I think that anyone who is depressed ought to eat more chocolate cake.” are those from personal experience. Phrases like “I know someone who…” or “In my experience, I’ve found…” should alert us to such evidence.

Because personal experiences are very vivid in our memories, we often rely on them as evidence to support a belief. For example, we may have a very bad experience with a car mechanic because we were greatly overcharged, leading us to believe that most car mechanics overcharge. While the generalizing about mechanics may or may not be true, relying on such experiences as the basis for a general belief is a mistake! It’s fallacious and is known as the Hasty Generalization Fallacy. The reason this is a mistake in reasoning is because a single experience or even an accumulation of personal experiences, is not enough to give us a representative sample of experiences.

Hasty Generalization Fallacy* 
- a person draws a conclusion about a large group based on experiences with only a few members of the group

A single experience or several such experiences can demonstrate that certain outcomes are possible, for example, you may have met several people who claim their lives were saved because they were not wearing their seat belts when they got into a car accident. Another example of this fallacy would be to charge that Christians are immoral or hypocritical because a few Christians bomb abortion clinics. Another example may be to charge that atheists are intolerant and oppressive because of the atrocities of atheist regimes such as Stalin’s. Such experiences, however, cannot demonstrate that such outcomes are typical or probable. We need to be wary when we hear someone arguing “Well, in my experience….

* I’ll address the Hasty Generalization Fallacy a bit more in evidence type #7, Research Studies.

Personal and anecdotal experience can be adequately used to support the possibility of something occurring, but not really its probability or typicality.

Problems: In general, very few people can have enough relevant experience with something that a factual claim about all of those things can be justified. Typically, personal experience is too limited to form the evidentiary basis for broad claims about an entire category of things (known as generalizations). Watch out for overgeneralizations (the fallacy known as hasty generalization) when offered personal experience as evidence to support a factual claim.

Exceptions: Personal experience is good evidence of what is possible. For instance, I never had good service at a Chevy dealership when I owned my Camaro. I had my car serviced at two different dealerships in two different states for a number of separate service issues. Does this mean that all Chevy dealerships have poor customer service? Probably not – to claim my experience is proof that one cannot get good customer service from Chevrolet is to commit the hasty generalization fallacy. However, my experience is proof that it is possible to experience poor customer service at a Chevy dealership.

The evidence is strong when: The evidence is weak when:
There is a significant amount of relevant experience under a variety of circumstances – this reduces the likelihood of overgeneralization. How much is sufficient depends on the issue or event. Personal experience is limited – too little, too few experiences. The less personal experience one has to go on, the less reliable the claim.
Personal experience is used to support a claim about what is possible. Personal experience is used to support a claim about what is necessarily true about an entire category of things.

ASK: Is there any other evidence to support this claim?

3. Testimonials 

Testimonials are a form of personal experience in which someone provides a statement supporting the value of some product, event, or service and the endorsement lacks any of the information we would need to decide just how much we should let it influence us.

Example: This book looks great. On the back cover, comments from readers say, “I could not put this book down.”

We often think of celebrity endorsements as testimonials, but if we look closer, we’ll see testimonials on numerous products such as books, cars, movies, skincare, and many more. Another example can be from students recommending a particular course or instructor (whether it be of a particular university or an online educational system).

Like #2 (Personal Experiences and Anecdotes) above, testimonials are usually not very helpful at all. In most cases, we should pay little attention to personal testimonials until we find out much more about the expertise, interests, values, and biases behind them. We should especially be wary of each of the following problems with testimonials.

 People’s experiences differ greatly. Those trying to persuade us have usually carefully selected the testimony they use. What we are most likely to see on the back of a book jacket is the bet praise, not the most typical reaction. We should always ask “What was the experience like for those whom we have not heard from?” in addition, the people who provide the testimonials have often been selective in their attention, paying special attention to information that confirms their beliefs and ignoring disconfirming information.Personal Interest. Many testimonials such as those for many products, come from people who have something to gain from their testimony. For example, drug companies often give doctors grants to do research, as long as they prescribe the drug company’s brands of medication. Thus, we need to ask, “Does the person providing the testimony have a relationship with what he is advocating such that we can expect a strong bias in his testimony?”Omitted Information. Testimonials rarely provide sufficient information about the basis for the judgement. For example, when a friend of yours encourages you to go see this new movie because it is the “best movie ever,” you should ask what about the movie is so impressive. Our standards for judgment may well differ from the standards of those given the testimony. We often have too little information to decide whether we should treat such claims seriously.The human factor. One reason that testimonials are so convincing is that they come from human beings and they are very vivid and detailed, a marked contrast to statistics and graphs, which tend to be very abstract. They are often provided by very enthusiastic people, who seem trustworthy, well-meaning, and honest. Such people make us want to believe them.

Testimonials, being a form of personal experience, are usually not a very good form of evidence and are thus, most likely not going to be very dependable in our evaluation of an argument.

Problems: Personal experience as testimonial is selective – advertisers will only select those who have had dramatically successful results or extraordinarily positive experiences with their product to tell their stories. Endorsers may be influenced by personal interest insofar as they may benefit by the successful sale of the product. When this is the case, they may be motivated to tell distorted versions of their experiences that are unreliable in order to promote sales and increase their own payoff. Finally, endorsers often omit information. In part, this is an extension of the personal interest problem – people will distort or delete information for personal gain, but there are other pressure on speakers to distort/delete as well – the desire to please those who have chosen us to give our testimonial, for instance (we know what they expect us to do and we are flattered to have been chosen above others

Exceptions: There really aren’t any – testimonials are usually “not very helpful at all” in helping us determine whether a claim is true. In short, it is very unreliable evidence.

The evidence is strong when: The evidence is weak when:
Never strong evidence, but when testimony is of experts whose interests, values, and goals are relevant and similar to yours, and when those endorsers are free from the bias that attaches to personal gain, it is something to consider – but with caution. It is overly selective.
N/A When testimonials are given by those who have a personal interest in the success of their pitch.
N/A When parts of endorser’s experience may be omitted, or when we are not allowed to hear about bad experiences.

ASK: “What do I know about the expertise, interests, values and biases of the individuals endorsing this product?”
ASK: “Who am I not hearing from?”
ASK: “How were endorsers chosen to give their testimonial?”
ASK: “Do they gain significantly by giving their testimonial?”
ASK: “What are they not telling me?”

4. Personal Observations

Personal observation is the kind of evidence that is given to support claims with significant authority and/or consequence in our society: scientific claims and legal claims. Claims backed by personal observation include claims about diet and nutrition, the benefits and dangers of taking certain medications, the causes and remedies of environmental hazards, the award of monetary damages (and orders to pay them), the guilt or innocence of an individual charged with a crime – sometimes leading to the execution of that individual.

Examples: (1) Witnesses of a car accident. (2) Players and referees of a football game.

Personal observations can be a valuable kind of evidence. It is often the basis of a the scientific approach. We feel confident in something we actually see more than something we hear from a 3rd party. Thus we tend to rely on witness testimony, perhaps with a bit of overconfidence. A difficulty with this form of evidence however, is that we have the tendency to see or hear what we wish to see or hear, selecting and remembering those aspects of an experience that are most consistent with our previous experience and background.

But observers do not give us “pure” observations. What we see and report is filtered through a set of values, biases, attitudes, and expectations. And because of these influences, observers often disagree about what they perceive. Thus, we should be wary of reliance on observations made by any single observer in situations in which we might expectant observations among observers to vary.

Here are a couple examples that should help illustrate the potential danger of relying on personal observation as evidence and are related to the previous set of examples.

(1) A player says he crossed the end zone and the referee says the player stepped out of bounds first.
(2) There is a car accident at a busy intersection. The drivers blame each other. Witnesses alternatively blame the drivers and a third car that sped off.

While personal observations can often be valuable sources of evidence, we do need to nevertheless recognize that they are not unbiased “mirrors of reality”; and when they are used to support controversial conclusions, we should seek verification by other observers as well as other kinds of evidence to support the conclusion. For example, if an employee claims that certain remarks from her boss are discriminatory, the claim is more credible if there are others who heard the remarks and also think the comments were discriminatory.

And when reports of observations are found in newspapers, magazines, books, television, and the internet are used as evidence, we need to determine whether there are good reasons to rely on such reports. The most reliable reports will be based on recent observations made by several people observing under optimal conditions who have no apparent, strong expectations or biases related to the event being observed.

Problems: Again, universal modeling processes and constraints give rise to a tendency to see or hear what we wish to see or hear, selecting and remembering those aspects of an experience that are most consistent with our previous experience and background. Several things reduce the reliability of personal observation:

  1. the number of corroborating observations – the fewer observers, or the fewer observations that agree, the less reliable the evidence;
  2. the conditions under which the event was observed – those observations made under optimal conditions are more reliable;
  3. the nature of the event under consideration – is it particularly controversial or sensitive so that strong previously established beliefs and values might distort the observation or lead to deletion or distortion in the retelling? And,
  4. how much time has passed between the observation (the event) and the retelling – the longer the time, the less reliable.

Exceptions: Recent observations made under optimal conditions by multiple observers who are relatively free of bias are generally reliable as evidence. Claims based on personal observation are, in fact, highly reliable; personal observations that meet these standards are among the best evidence one can give to support the truth of their claim.

The evidence is strong when: The evidence is weak when:
Observations are recent. Observations are retold long after the event (more time for generalizing to work on distorting memory and for events to be lost to deletion).
When the observation is confirmed by multiple observers. When only one observation is available (only one person witnessed the event) or when there are multiple conflicting accounts of the event.
When the conditions under which the event was observed were optimal (few to no obstructions or distortions due to angle, weather, or other environmental constraints). When the conditions under which the event was observed would have made accuracy difficult – if observation was obstructed or delayed or otherwise impeded.

ASK: When did the event this person observed occur?
ASK: Under what conditions was this observation made?
ASK: Did anyone else observe the incident? How similar (or different) are the accounts?

5. Case examples

Case examples are detailed descriptions that are usually based on observations or interviews (and vary from being in-depth and thorough to being superficial) and are used to support a conclusion.

Example: President of a large university: “Of course our students can move on to high paying jobs and further study at large universities. Why, just this past year we sent one of our students, Joe Bob Puddinpants, off to law school at Harvard. In his first year Joe Bob remained in the top five percent of his class. Therefore, our students can certainly achieve remarkable success at elite universities.”

Argument makers often begin persuasive presentations with dramatic descriptions of cases. For example, one way to try to persuade others about the dangers of drunk driving are to offer heart-wrenching stories of young people dying in car accidents where the driver had been consuming alcohol.

Case examples are often compelling to us because of their colorfulness and their interesting details, which make them easy to visualize. Political candidates have increasingly resorted to case examples in their speeches, knowing that the rich details of cases generate an emotional reaction. But such cases should be viewed more as striking examples or anecdotes than as proof, and we must be very suspicious of their use as evidence.

Dramatic cases appeal to to our emotions and distract us from seeking other more relevant research evidence. For example, imagine a story about a man who tortured and murdered his numerous victims. The emotions triggered by such a story are likely to increase our desire for capital punishment. Yet, the human drama of these crimes may lead us to ignore the fact that such a case is rare and that over the past 30 years, 119 inmates with capital sentences were found to be innocent and released from prison. Moral: Be wary of striking case examples as proof!

Although case examples may be consistent with a conclusion, do not let that consistency fool you. Always ask ”Is the example typical?” “Are there powerful counterexamples?” “Are there biases in how the example is reported?”

Problems: Case examples are anecdotal evidence chosen for its vivid detail and ability to invoke emotional responses in listeners. They are selective in the same way that testimonials are – we are likely to be presented only with cases chosen for their ability to produce the desired reaction, to persuade us of the truth of the claim, and not likely to be presented with cases that weaken the claim in need of support. Beware of case examples as they are often disguised examples of appeals to emotion. In addition, case examples, like personal experiences, demonstrate important possibilities and put a personal face on abstract statistics. They make it easier for people to relate to an issue and thus, take more interest in. However, they do not address the issue of probability.

Exceptions: When case examples are presented as evidence of what is possible. The case example presented in the first example (about Joe Bob Puddinpants) proves only that is possible for students of that university to go on to great things – but it proves nothing about what is necessary or even likely.

The evidence is strong when: The evidence is weak when:
The claim it is given to support is a claim about what is possible. The claim is about what is likely or necessary, what is typical.
There are multiple cases in which the same thing occurred – lots of corroborating examples – these outnumber the opposite cases. There are multiple counterexamples – enough counterexamples to undermine the claim that such cases are typical, the norm, or what is to be expected.

ASK: “How typical is this example?” “What counterexamples am I not being told about?”

6. Appeals to Authority - coming soon

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