Flaw in Reasoning questions ask you to name the specific logical error in an argument. Unlike Weaken questions, you are not introducing new information — you are diagnosing what is already wrong with the reasoning as presented.
A) Identifying the Question Type
- "The argument is flawed because it…"
- "The reasoning above is most vulnerable to criticism on the grounds that it…"
- "Which of the following identifies a flaw in the reasoning?"
- "The argument fails to consider…"
B) How to Approach Every Flaw Question
- Map the argument: identify the conclusion and every premise, one sentence at a time.
- Find the gap: ask "Does the conclusion follow necessarily from the premises? What step is unjustified?"
- Name the flaw before reading choices: "This is a correlation/causation error" or "This treats necessary as sufficient." Having a label makes matching faster.
- Match the abstract description: correct answers describe the flaw in abstract logical terms, not specific content terms from the stimulus.
C) Core Flaw Types
1. Correlation Treated as Causation
The argument observes A and B together and concludes A causes B — without ruling out alternatives, reverse causation, or coincidence. Most frequently tested flaw on the GMAT.
Answer choice phrasing: "treats a correlation between two factors as evidence that one causes the other" / "fails to consider that the relationship may have the opposite causal direction."
2. Hasty Generalization (Unrepresentative Sample)
Generalizes from a sample too small, self-selected, or unrepresentative of the broader population.
Answer choice phrasing: "draws a conclusion about all X based on evidence from a subset that may not be representative" / "relies on data that may not be typical of the broader population."
3. False Dichotomy (Either/Or Fallacy)
Presents exactly two options as if they are the only possibilities, when other alternatives exist and have not been ruled out.
Answer choice phrasing: "assumes without justification that there are only two possible outcomes" / "overlooks alternatives other than the two presented."
4. Circular Reasoning (Begging the Question)
The conclusion is essentially restated as one of the premises — the argument assumes what it is trying to prove.
Answer choice phrasing: "uses as a premise a claim essentially equivalent to its conclusion" / "assumes the truth of what it is attempting to prove."
5. Faulty Analogy
Draws a conclusion by comparing two situations as relevantly similar when they differ in ways that affect the conclusion.
Answer choice phrasing: "draws an analogy between two situations that may not be relevantly similar" / "applies a conclusion from one context to a different context without justification."
6. Necessary vs. Sufficient Condition Confusion
Treats a necessary condition (required but not enough alone) as if it were sufficient (enough by itself to guarantee the outcome).
Example: "To get into a top MBA program, you need a strong GMAT score. Maria has a strong GMAT score. Therefore Maria will get in." — Score is necessary, not sufficient.
Answer choice phrasing: "treats a condition necessary for X as though it were sufficient to guarantee X."
7. Composition / Division Fallacy
Composition: assumes what is true of each part is true of the whole. Division: assumes what is true of the whole applies to each part.
Answer choice phrasing: "assumes a property of each component must apply to the whole" / "concludes that because the whole has a property, each element must too."
8. Ad Hominem (Attacking the Source)
Dismisses a claim by attacking the person or institution rather than evaluating the argument on its merits.
Answer choice phrasing: "rejects a conclusion based on its source rather than on the evidence it provides" / "dismisses a claim on the basis of who made it rather than its logical merit."
9. Straw Man
Misrepresents an opponent's position by exaggerating, distorting, or simplifying it, then attacks the distorted version rather than the actual argument.
Answer choice phrasing: "attacks a position that was not actually argued" / "characterizes the opposing view inaccurately and then refutes that mischaracterization."
10. Equivocation
Uses the same word with two different meanings in the same argument, creating a false logical connection. The conclusion relies on a shift in the meaning of a key term.
Answer choice phrasing: "relies on a key term being used in two different senses" / "conflates two distinct meanings of the same word to bridge premise and conclusion."
11. Appeal to Authority
Cites an expert or authority figure as evidence for a conclusion, but the authority's expertise is not relevant to the specific claim being made.
Answer choice phrasing: "treats the endorsement of an authority in one domain as evidence for a claim in a different domain" / "relies on testimony from someone whose expertise does not apply to the conclusion."
12. Temporal Fallacy (Post Hoc)
"After X, Y happened, therefore X caused Y." Assumes causation solely because one event followed another in time — no actual causal mechanism is established.
Answer choice phrasing: "treats temporal sequence as evidence of a causal relationship" / "concludes that an earlier event caused a later one merely because it preceded it."
13. Ignoring Opportunity Cost
Evaluates a plan or decision without considering what alternatives were given up. The argument concludes a plan is good without comparing it to the next-best option.
Answer choice phrasing: "fails to consider what other uses could have been made of the resources" / "evaluates the outcome of a decision without accounting for the alternatives foregone."
14. Survey / Self-Selection Bias
Draws broad conclusions from a survey whose respondents self-selected or whose sample is systematically unrepresentative — those most likely to respond differ from the population.
Answer choice phrasing: "relies on a survey whose respondents may not represent the broader population" / "draws a general conclusion from data collected in a way that attracts atypical participants."
D) Numerical Reasoning Traps (Common in CR)
Many GMAT CR arguments involve statistics or numbers. These traps are distinct from pure logical fallacies but appear frequently on Weaken, Flaw, and Strengthen questions.
- Percent vs. absolute number: "Crime rate rose 10%" does not mean 10% more crimes occurred if the population changed. A higher percentage with a smaller base can mean fewer events in absolute terms — and vice versa.
- Total vs. per-capita: a large total count may hide a small per-person figure. "Country X has more doctors than Country Y" is misleading if X has ten times the population.
- Rate vs. count: a rising rate with a shrinking base can mean fewer actual cases. "The rate of product defects tripled" paired with a 90% production cut could mean fewer total defective units.
- Base rate neglect: if a disease is rare, even a highly accurate test will produce many false positives. Arguments about diagnostic or predictive accuracy often ignore the prior probability of the condition.
E) Flaw vs. Weaken: The Key Distinction
- Flaw: diagnoses the internal logical error — what is structurally wrong with the reasoning as already given. No new information. Answer describes the error type abstractly.
- Weaken: introduces a new fact (accepted as true) that reduces the conclusion's likelihood. The argument's structure may be valid; the new fact just undermines it.
Think of it this way: Flaw is a structural diagnosis. Weaken is a factual counter-argument.