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4.4: Decide How Strong the Evidence Is

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    Is there enough evidence?

    An argument may not have obvious exceptions, but we should still ask whether it gives enough evidence to convince us of the claim.  The reason is the foundation of the argument: is the foundation secure? In some cases we may suspect that some part of the reason is not true, and in others we may simply want to note that little or no evidence has been offered.  For example, the border argument we have examined makes two claims without any evidence: “Completely open borders would put our security at risk” and “There are ways to regulate the border without criminalizing people." 

    The evidence may lack substance (circular reasoning)

    Sometimes a reason given is not really a reason at all, just a repetition of the claim itself in different words. In effect, the writer asks us to believe an idea because of that very same idea. This is called circular reasoning or "begging the question."

    For example, consider the following argument:

    Anyone born in the United States has a right to citizenship because citizenship rights here depend on birth, not ethnicity or family history of immigration.

    The idea that “anyone born in the United States has a right to citizenship” and the idea that “citizenship rights here depend on birth” are really one and the same. We still need a reason to accept this focus on birth as the determining factor.

    Circular reasoning is often not deliberate. In reaching to explain the reason for a deeply held belief, a writer may end up summarizing that belief again in a different way. Other times the writer may knowingly perform this sleight of hand, hoping the reader will not notice. In either case, the argument lacks substantive support.

    We can critique circular reasoning with phrases like the following:

    • The argument presents _____________ as a reason to believe _____________, but this supposed reason is just a rewording of the claim.

    • The writer provides no real justification for the idea that _____________; to convince us they just repeat that idea with different phrasing.

    The evidence may not be representative (hasty generalization)

    Most academic arguments are inductive; they explore evidence in the form of specific examples, facts, statistics, testimonials, or anecdotes in order to arrive at a general conclusion. If an argument does give relevant evidence, we need to know if it is enough.  A few examples may not be representative of a general pattern. If the argument makes a sweeping generalization based on one or two anecdotes or solely on the writer's own experience, it can be considered a hasty generalization. 

    How do we decide when the evidence is enough? The science of statistics addresses this question in very specific, technical ways that are worth learning but beyond the scope of this book. Often, however, an intuitive assessment will be enough. We all probably already guard against this fallacy when we search online for products that have been reviewed many times. Clearly one five-star review could be a fluke, but 2,000 reviews averaging 4 1/2 stars is a more reliable indicator.

    People who deny that global warming is a genuine phenomenon often commit this fallacy. In February of 2015, the weather was unusually cold in Washington, DC. Senator James Inhofe of Oklahoma famously took to the Senate floor wielding a snowball. “In case we have forgotten, because we keep hearing that 2014 has been the warmest year on record, I ask the chair, ‘You know what this is?’ It’s a snowball, from outside here. So it’s very, very cold out. Very unseasonable.”

    Senator Inhofe commits the hasty generalization fallacy. He’s trying to establish a general conclusion—that 2014 wasn’t the warmest year on record, or that global warming isn’t really happening. But the evidence he presents is insufficient to support such a claim. His evidence is an unseasonable coldness in a single place on the planet, on a single day. We can’t derive from that one example any conclusions about what’s happening, temperature-wise, on the entire planet, over a long period of time. The claim that the earth is warming is not a claim that everywhere, at every time, it will always be warmer than it was. The claim is that, on average, across the globe, temperatures are rising. Cold snaps can happen even if temperatures are rising.

    A particularly damaging example of the hasty generalization fallacy is the development of negative stereotypes. Stereotypes are general claims about religious or racial groups, ethnicities and nationalities. Even if we do have evidence that a certain trait is more common among people of one ethnicity, we still cannot assume that a particular individual of that ethnicity will have the trait.

    A specific form of hasty generalization is when an author will point to a lack of evidence as a sign that no evidence is out there. This fallacy is often called appeal to ignorance because the arguer is citing their own lack of knowledge as the basis for their argument

    For example, consider the following: “No one I know has heard about any anti-Asian violence lately; therefore, reports of such violence are exaggerated.”  The speaker and their acquaintances may simply not have come in contact with the people who have experienced such incidents of violence. 

    Absence of evidence can sometimes tell us something useful. It may be a reason to doubt the conclusion even if it doesn't disprove it. During the 2016 presidential campaign, reporter David Fahrenthold took to Twitter to announce that despite having “spent weeks looking for proof that [Donald Trump] really does give millions of his own [money] to charity...” he could only find one donation, to the NYC Police Athletic League. Trump has claimed to have given millions of dollars to charities over the years. Does this reporter’s failure to find evidence of such giving prove that Trump’s claims about his charitable donations are false? No. To draw such a conclusion relying only on this reporter's testimony would be to commit the fallacy.

    However, the failure to uncover evidence of charitable giving does provide some reason to suspect Trump’s claims may be false. How much of a reason depends on the reporter’s methods and credibility, among other things. In fact, Fahrenthold subsequently performed and documented in the Washington Post on 9/12/16 a rather exhaustive unsuccessful search for evidence of charitable giving, providing strong support for the conclusion that Trump didn’t give as he’d claimed.

    Is the evidence trustworthy? 

    If the writer has offered evidence, we should ask ourselves whether it is credible. Can it be verified? The validity depends on the source. Is the evidence from trustworthy sources?  For example, if the argument cites a statistic from the Pew Research Center, we need to know whether that institution is credible.  Is it biased?  Does it try to promote a particular product or ideology? Do experts in the field review its studies? If we are not familiar with the source, we can look it up online and include this information in our assessment. We will discuss assessing the credibility of sources much more in Chapter 6 on research and also in Chapter 9 on how arguments build trust.

    Is there enough variety in the evidence? 

    There are different kinds of evidence and each kind has its limitations as far as what it can show.  Thus, arguments are often most convincing when they provide a variety of kinds of evidence. For example, an anecdote might give a sense of how difficult an immigrant's situation in their country of origin may be, but a statistic on how common that difficulty is will be needed to show that the anecdote is typical of many others' experiences as well. In your assessment, you may want to note the limitations of the evidence offered and point out another kind of evidence that would complement it. Are there enough statistics, anecdotes, or testimonials? Is there enough variety in the kinds of evidence?  There isn’t a set formula for what’s needed; the question is whether the readers should be convinced that any claim that makes up part of the argument is valid.

    Types of evidence and their limitations


    Facts are statements that can be independently verified. For example, an argument might state that “According to the Pew Research Center, the United States has more immigrants than any other country.” We could theoretically check whether the Pew Research Center issued this statement and also check whether it is true based on the census of each country as well as other population estimates.  

    Statistics Edit section

    Statistics are numbers that are used to describe a pattern.  They often represent information about a large number of cases of a given phenomenon, so they may be more convincing because they are more likely to represent a general trend than one or two cases might be.  If the statistics are accurate and relevant, they can provide strong support.  For example, an argument might cite evidence that according to the Pew Research Center, “immigrants today account for 13.6% of the U.S. population.”  Statistics have an air of authority because they quantify things, making them and the claim they support sound indisputable. For this reason, writers can be tempted to overuse them or to throw them in where they don’t really add to the logic of the argument.  A famous book called How to Lie with Statistics by Darrell Huff goes over all the ways in which statistics can be used to mislead readers about the strength of a claim. We need to examine closely what a given statistic actually shows and exactly how it connects to the claim at stake in the argument. This will usually involve checking the assumptions made to link the statistic to the claim, as we will see in Section 4.5: Check the Argument’s Assumptions (link).

    Expert testimonials Edit section

    Testimonial evidence can be convincing if it is collected from relevant authorities. Whether or not a testimonial is convincing depends not just on how well regarded the expert is but on how relevant their expertise is to the topic at hand. Who would be an expert source of a testimonial for an argument based on immigration? A social scientist?  A philosopher? An immigration lawyer? We would want to question the testimony of a celebrity who has no special knowledge of immigration.  In addition, we want to know if the expert’s perspective is representative of the opinion of others in the field.  Is the person an extremist?  Do they have a stake in promoting a particular product or position?

    Statements from experts or organizations that represent a field of knowledge can be especially helpful in laying the foundation for a deductive argument, where we need a credible general principle as the foundation for a conclusion about a specific case. This can be especially helpful if we are looking to make a prediction about a future trend or the outcome of an experiment.  We will need to cite experts to substantiate the general principle. But the question arises whether the experts really speak for the field and whether others have alternate expert interpretations of the pattern or draw other generalizations from the body of evidence.

    For example, take the following general claim supported by expert testimony:

    As psychiatrist Dr. Robert Spitzer of Columbia University told The Washington Post in 2001 that his study showed that, "some people can change from gay to straight, and we ought to acknowledge that." It’s not impossible to convert to heterosexuality.

    However, an inquiry into Dr. Robert Spitzer will show that his 2001 study was widely criticized by other psychiatrists and that he himself recanted the study and apologized for it in the journal of the American Psychiatric Association in 2012, writing, “I … apologize to any gay person who wasted time and energy undergoing some form of reparative therapy because they believed that I had proven that reparative therapy works with some ‘highly motivated’ individuals.”  An assessment could note that at the very least the argument should have mentioned this later apology when it quoted Spitzer.

    Anecdotes Edit section

    Anecdotes can illustrate a point with a story that makes it come to life. They are more compelling if they are based on first-hand accounts. Often these stories appeal strongly to readers' emotions, and we will discuss in much more depth how to analyze and evaluate these appeals in Chapter 8: How Arguments Appeal to Emotion. We should examine any story closely to see how opinions and assumptions may be woven into the storytelling.  In our assessment, we may want to point out any possible bias or limitation of the person who provides the anecdote.  

    If anecdotes or specific examples are used in inductive reasoning to establish a general pattern, we can ask how the argument convinces us that these are typical. Sometimes statistics can help to establish this typicality. 

    Does the evidence really support the claim?

    The argument may have offered some facts as evidence, and we may be ready to accept them as facts, but do they prove what the argument wants them to prove?  Sound evidence can be used in misleading ways.  As we’ll see in Section 4.5: Check the Argument’s Assumptions (link), a reason depends on assumptions to prove a claim.  If the assumptions are wrong, then the reason does not really prove the claim.  This type of fallacy, or logical problem, can be called a non sequitur.


    Is the claim too broad or too definite given the evidence?  

    Sometimes an argument makes a broad claim based on narrow evidence.  In our assessment, we can comment on any mismatch between the scope of the claim and the scope of the evidence offered. We might suggest that the argument should limit its claim with a particular phrase such as with “few,” “many,” “most,” “some,” or “in a few cases.” 

    Sometimes an argument makes a bold, absolute claim, but the evidence really only justifies a more tentative conclusion. We can point out in our assessment that the argument lacks the appropriate qualifying words like “possibly,” “maybe,” “probably,” “almost certainly,” or “in all likelihood.”

    See Section 2.8: Finding the Limits on the Argument (link) for more ways to limit the scope or degree of certainty.

    Phrases for assessing an argument’s evidence

    Praising evidence

    • She convincingly supports this claim by _____________.

    • They give many examples of _____________ to support the idea that _____________.

    • His evidence of _____________ ranges from anecdotes to large-scale academic studies to expert testimonials.

    • X refers to credible academic studies of _____________ to bolster their argument that _____________.

    • X refers to a number of credible experts to establish that, in general, _____________.

    Critiquing evidence Edit section

    • X asserts that _____________ but does not offer any evidence.

    • The argument builds on the premise that _____________, but fails to support that premise.

    • X offers scant evidence for the claim that _____________.

    • The argument gives an example to support the claim that _____________, but gives no evidence that this example is typical.

    • _____________ is not enough to show that _____________.

    • The essay offers only _____________ as evidence when it should also point to _____________ and _____________.

    • X’s claim that _____________ is too broad given that they only give evidence related to _____________.

    • The evidence does not warrant such a definite conclusion about _____________.

    • X has been a bit hasty to declare that_____________. So far, the scanty data on _____________ only warrant cautious speculation.

    Exercise \(\PageIndex{1}\)

    Choose an argument you have recently read for class, or select one from Section 12.1: Suggested Short Readings.  Make a list of the pieces of evidence the argument presents and decide whether each piece is a fact, statistic, testimony, or anecdote.  Is a credible source given for each?


    Most of the above is original content by Anna Mills, licensed under CC BY NC. The section on types of evidence is by Tina Sander. The description of the hasty generalization and appeal to ignorance fallacies are adapted from the "Informal Logical Fallacies" chapter of Matthew Knachel's Fundamental Methods of Logic, licensed CC BY.