CR – Flaw in Reasoning

What is Your Mission?

Your goal is to find the answer choice that describes the logical error *already* present in the argument.

You are not adding new information. You are acting as a diagnostician, pointing out the *reason* why the conclusion does not logically follow from the premises.

Analogy: The Broken Chain

An argument is a chain linking premises to a conclusion. A flaw is a broken link.

PREMISE 1

PREMISE 2

THE FLAW

CONCLUSION

Your job isn't to *break* the chain (Weaken) or *fix* it (Strengthen). Your job is to *point to the broken link* and say, "That's what's wrong."

The #1 Point of Confusion: Flaw vs. Weaken

These are related but *different* tasks.

WEAKEN Question

The Task: *ADD* a new fact to hurt the argument.

The Answer: Is a *new piece of evidence*.

Question: "Which of the following, *if true*, most weakens...?"

Answer: "A recent study showed that a competitor's bankruptcy was the real cause of the profit increase."

FLAW Question

The Task: *DESCRIBE* the error already in the argument.

The Answer: Is a *description of the reasoning*.

Question: "The argument's reasoning is flawed because it..."

Answer: "...mistakes a temporal sequence for a causal relationship." (or "...fails to consider alternative causes...")

Decoding the Question Stems

They all ask: "What's wrong with this logic?"

"The argument's reasoning is flawed because it..."
"The argument is vulnerable to criticism on the grounds that it..."
"The reasoning in the argument is questionable because it..."
"The argument fails to consider the possibility that..."
"The argument makes which of the following errors in reasoning?"

The Core Concept: A Flaw is an Invalid Assumption

Every logical flaw can be rephrased as an unjustified assumption the author made.

Example: The "Expert" Argument

Argument: "Dr. Smith, a Nobel Prize-winning physicist, says that the 'Quantum' brand of multivitamins is the best. Therefore, we should all take 'Quantum' vitamins."

The FLAW is...

"The argument appeals to an authority who is not an expert in the relevant field."

Which means the INVALID ASSUMPTION was...

"The author assumes that a physicist is also an expert in nutrition."

Finding the flaw *is* finding the argument's most ridiculous, unjustified assumption.

The Hall of Flaws

This is your "Most Wanted" list. Almost every flaw question on your exam will feature one of these 16 logical errors.

The 16 Most Common Flaws

1. Correlation vs. Causation 2. Post Hoc Flaw 3. Sampling / Generalization 4. Flaw of Analogy 5. Ad Hominem 6. Straw Man 7. Circular Reasoning 8. False Dilemma 9. Slippery Slope 10. Appeal to Authority 11. Appeal to Popularity 12. Part-to-Whole 13. Whole-to-Part 14. Necessary vs. Sufficient 15. Sufficient vs. Necessary 16. Equivocation

Flaw #1: Correlation vs. Causation

The Mistake: Assuming that because two things happen at the same time (a correlation), one must be *causing* the other.

The Argument: "A study shows that people who own yachts have lower rates of heart disease. Therefore, buying a yacht is good for your heart."

The Flaw: "The argument fails to consider a third factor—in this case, wealth. Wealthy people can afford both yachts and high-quality healthcare, which is the real reason for better heart health."

Flaw #2: Post Hoc (Temporal) Flaw

The Mistake: A variation of the causation flaw. Assuming that because A happened *before* B, A must have *caused* B.

The Argument: "We installed a new air filter, and two days later, my cold was gone. The air filter must have cured my cold."

The Flaw: "The argument overlooks the possibility that the cold simply ran its natural course. It mistakes a temporal sequence for a causal relationship."

Flaw #3: Sampling / Generalization

The Mistake: Drawing a broad conclusion based on a sample that is too small, biased, or unrepresentative.

The Argument: "I asked five people at the gym if they like the new protein shake, and all five loved it. Therefore, the new shake is a huge success with all our customers."

The Flaw: "The argument draws a general conclusion about *all customers* based on a small, *biased sample* of gym-goers, who are not representative of the general population."

Flaw #4: Flaw of Analogy

The Mistake: Assuming that because two things are similar in *one* way, they must be similar in *another, relevant* way.

The Argument: "Just like a car, the human body needs a 'tune-up' every 5,000 miles. We should all get a full medical check-up after every 5,000 miles we run."

The Flaw: "The argument relies on an analogy between a mechanical car and a biological body, which may not be similar in ways that relate to maintenance or running."

Flaw #5: Ad Hominem (Attack on Person)

The Mistake: Attacking the *person* making the argument (their character, motives) instead of the *argument itself*.

The Argument: "Dr. Evans' new climate theory is absurd. He's a terrible public speaker and is known to be rude to his colleagues."

The Flaw: "The argument rejects a theory based on the personal characteristics of the person who proposed it, rather than on the *merits* of the theory itself."

Flaw #6: Straw Man

The Mistake: Misrepresenting or exaggerating an opponent's argument to make it easier to attack.

The Argument: "My opponent wants to increase funding for public parks. Clearly, he wants to bankrupt the city by spending *all* our money on trees and squirrels!"

The Flaw: "The argument mischaracterizes the opponent's modest proposal (increase funding) as an extreme one (spend *all* money) and then attacks that extreme position."

Flaw #7: Circular Reasoning

The Mistake: The conclusion is just a restatement of the premise. The argument assumes what it's trying to prove.

The Argument: "This novel is the best-selling book of the year because it has sold more copies than any other book."

The Flaw: "The argument's premise ('sold more copies') and its conclusion ('best-selling') are logically identical. It provides no *reason* for the book's success."

Flaw #8: False Dilemma (Either/Or)

The Mistake: Presenting only two options as the *only* possibilities, when in fact other options exist.

The Argument: "We must either cut school funding or our city will go bankrupt. Therefore, we must cut school funding."

The Flaw: "The argument fails to consider other possibilities, such as raising taxes, cutting other programs, or improving efficiency."

Flaw #9: Slippery Slope

The Mistake: Claiming that a small, reasonable first step will inevitably lead to a chain of extreme, negative events.

The Argument: "If we allow the city to ban plastic bags, next they'll ban plastic bottles, then plastic straws, and soon all plastics will be illegal!"

The Flaw: "The argument claims, without evidence, that a moderate first step will *necessarily* lead to a series of extreme, related consequences."

Flaw #10: Appeal to Authority

The Mistake: Relying on the opinion of an "expert" who is either not an expert at all, or is speaking outside their field of expertise.

The Argument: "My dentist, a top expert in his field, says that this new car has the best engine. So, I'm buying it."

The Flaw: "The argument relies on the testimony of an authority (a dentist) who is not an expert in the relevant subject (automotive engineering)."

Flaw #11: Appeal to Popularity

The Mistake: Assuming something is true, good, or right simply because many people believe it is.

The Argument: "Millions of people have bought this new diet book, so it must be a safe and effective way to lose weight."

The Flaw: "The argument equates the popularity of an item with its quality, which is not a logically sound connection."

Flaw #12: Part-to-Whole (Composition)

The Mistake: Assuming that what is true of the *parts* of something must also be true of the *whole*.

The Argument: "Every musician in this orchestra is a world-class soloist. Therefore, this orchestra must be the best in the world."

The Flaw: "The argument assumes that a team (the orchestra) will have the same quality as its individual members, ignoring factors like teamwork and direction."

Flaw #13: Whole-to-Part (Division)

The Mistake: The reverse of #12. Assuming that what is true of the *whole* must also be true of *each individual part*.

The Argument: "Our company is one of the most profitable in the country. Therefore, every employee in our company must be highly profitable."

The Flaw: "The argument assumes that a property of the whole (profitability) applies to each of its individual parts (the employees)."

Flaw #14: Mistaking Necessary for Sufficient

The Mistake: "You need X to get Y. I have X. Therefore, I will get Y." (X is *necessary*, but may not be *enough*).

The Argument: "To get into this law school, you need a high LSAT score. I got a very high LSAT score, so I will definitely be accepted."

The Flaw: "The argument overlooks other necessary factors, such as grades, essays, and letters of recommendation. It mistakes a necessary condition for a sufficient one."

Flaw #15: Mistaking Sufficient for Necessary

The Mistake: "Getting X is one way to get Y. I don't have X. Therefore, I can't get Y." (X is *sufficient*, but not the *only* way).

The Argument: "Drinking 10 cups of coffee is a way to stay awake. I didn't drink 10 cups of coffee, so I will fall asleep."

The Flaw: "The argument fails to consider other ways to stay awake (e.g., getting a good night's sleep). It mistakes a sufficient condition for a necessary one."

Flaw #16: Equivocation (Ambiguity)

The Mistake: Using a key word or phrase with two different meanings in the same argument, as if it has only one.

The Argument: "The scientist said she has a *theory* about the new particle. But a theory is just a guess. Therefore, the scientist is just guessing."

The Flaw: "The argument uses the word 'theory' in two different senses: first in the scientific sense (a well-substantiated explanation) and second in the casual sense (a guess)."

The 4-Step Strategy to Victory

1

Isolate Premise & Conclusion. Clearly state: "The author is using [Premise] to prove [Conclusion]."

2

Find the Gap. Ask yourself: "Does the premise *really* prove the conclusion? What's missing?" This is the *invalid assumption*.

3

Pre-think the Flaw. Put the invalid assumption into words. (e.g., "The author is assuming a sample of 5 people represents everyone.")

4

Match the Description. Find the answer choice that *describes* the flaw you just pre-thought. (e.g., "draws a broad conclusion from a small sample.")

The Hall of Shame: Common Traps

Don't fall for these tricky wrong answers.

1. The "Weaken" Trap

This is the #1 trap. The answer *adds new information* to hurt the argument, but it doesn't *describe the flaw itself*. (e.g., "A new study showed...")

2. The "Disagree" Trap

The answer just disagrees with the *conclusion* or *premise* without explaining the *logical error* that connects them.

3. The "Mismatched Flaw"

The answer describes a *real* logical flaw (like a "Straw Man"), but it's not the flaw the argument *actually* committed (which was a "Causation" flaw).

4. The "Too Strong / Too Weak"

The answer is too extreme ("The argument proves *nothing*") or too mild ("The argument mentions a detail..."). It must match the severity of the error.

Practice Set 1 (4 Options)

Apply the 4-step strategy. Find the flaw.

Passage 1: "The new 'Smart' brand lightbulb is clearly of the highest quality. We know this because it is the most expensive lightbulb on the market. Therefore, it will last the longest."

The argument is flawed because it...

Explanation:
Premise: Most expensive bulb. Conclusion: Highest quality / lasts longest.
Invalid Assumption: The author *assumes* that price = quality.
(A) is irrelevant (brightness isn't mentioned). (C) is a "Weaken" trap (new info). (D) is false (no Ad Hominem).
(B) Correct. This answer perfectly *describes* the invalid assumption (the broken link) in the argument.

Passage 2: "My opponent, Mr. Jones, argues that we should invest in a new public library. But Mr. Jones was fined last year for failing to pay his property taxes. Clearly, his arguments about public spending are not to be trusted."

The reasoning in the argument is most vulnerable to criticism on the grounds that it...

Explanation:
This is a classic Ad Hominem (Flaw #5). The argument doesn't address the library proposal at all. It just attacks Mr. Jones's personal character (tax issues).
(A), (C) are Mismatched Flaws. (B) is true, but (D) is the *specific logical error* the argument commits.
(D) Correct. This is the textbook definition of an Ad Hominem flaw.

Practice Set 2 (5 Options)

Passage 3: "Every member of our championship-winning soccer team is a vegetarian. This just proves that a vegetarian diet is the key to athletic success."

The argument's reasoning is flawed because it...

Explanation:
This is a Correlation vs. Causation (Flaw #1). The argument sees a correlation (vegetarian diet, success) and assumes causation (diet *caused* success).
(A) Correct. This answer points out the flaw: the argument "overlooks... other factors," which is the classic description of a causation flaw.
(B) is a Straw Man. (C) and (D) are irrelevant. (E) is tempting (Sampling Flaw), but the *primary* error is causal. The team is the whole sample, but the *reason* for their success is the flawed link.

Passage 4: "The head of the teachers' union stated that class sizes must be reduced to improve education. But this is wrong. You can't just throw money at the school system and expect students to get smarter."

The speaker's reasoning is most questionable because it...

Explanation:
This is a Straw Man (Flaw #6). The union's proposal is specific ("reduce class sizes"). The speaker misrepresents this as a vague, wasteful plan ("just throw money at the system") and attacks that misrepresentation.
(A) is false; the speaker *attacks* the authority. (B) is a flaw, but not the one committed.
(C) Correct. This answer perfectly *describes* the Straw Man flaw.

Final Interactive Quiz

Test your mastery. Diagnose the flaw.

Summary & Next Steps

Key Takeaways

Flaw = Describe. Weaken = Add. This is the most important distinction.
A flaw is just an unjustified, invalid assumption.
Your answer choice will be a description *of* the argument, not a new fact.
Memorize the "Hall of Flaws." 90% of questions use one of them.

Practice Plan

This is the most "academic" CR type. Spend time learning the 16 flaw names.
When you pre-think, *name the flaw*. (e.g., "Ah, that's a Part-to-Whole flaw.")
Do practice sets of *only* Flaw and Weaken questions back-to-back to master the difference.

Flaw in Reasoning