Identify a Flaw: LSAT Logical Reasoning Worked Examples
Master the art of spotting logical flaws with detailed examples, proven strategies, and official LSAC resources to boost your LSAT score.
Understanding LSAT Flaw Questions
Flaw questions are among the most common question types in the LSAT Logical Reasoning section, typically accounting for 15-20% of all Logical Reasoning questions. These questions test your ability to identify specific errors in reasoning that prevent an argument's conclusion from being fully supported by its premises.
Unlike "weaken" or "strengthen" questions where you evaluate additional information, flaw questions ask you to critique the internal logic of the argument as presented. You'll recognize these questions by stems such as:
- "Which one of the following most accurately describes a flaw in the argument's reasoning?"
- "The reasoning in the argument is most vulnerable to criticism on the grounds that..."
- "A flaw in the argument is that the argument..."
- "The argument is questionable because it..."
15 Most Common Logical Flaws on the LSAT
1. Correlation vs. Causation
The Flaw: Concluding that because two events occur together or are correlated, one must cause the other. This ignores alternative explanations such as reverse causation, common causes, or mere coincidence.
Example: "Studies show that people who drink coffee regularly have higher rates of heart disease. Therefore, coffee causes heart disease."
Why it's flawed: Perhaps people with stressful jobs (which increase heart disease risk) drink more coffee to stay alert. The correlation doesn't prove causation.
2. Confusing Necessary and Sufficient Conditions
The Flaw: Treating a necessary condition as if it were sufficient (or vice versa). If A → B, then B is necessary for A, but having B doesn't guarantee A.
Key Distinction:
• Sufficient: If you have A, you definitely have B (A is enough for B)
• Necessary: You must have B to have A, but B alone isn't enough (B is required for A)
Example: "To become a lawyer, you must pass the bar exam. John passed the bar exam, so John will become a lawyer."
Why it's flawed: Passing the bar is necessary but not sufficient. John also needs a law degree, character review approval, and must choose to practice law.
3. Sampling Error / Overgeneralization
The Flaw: Drawing a broad conclusion from a sample that is too small, unrepresentative, or biased. The argument assumes the sample accurately reflects the entire population.
Example: "I surveyed 20 students from the honors program and found that 90% plan to attend graduate school. Therefore, most students at this university plan to attend graduate school."
Why it's flawed: Honors students are not representative of all students. The sample is biased toward high-achieving students more likely to pursue advanced degrees.
4. False Dichotomy (False Dilemma)
The Flaw: Presenting only two options when other alternatives exist. The argument assumes "either A or B" when C, D, or E might also be possible.
Example: "Either we raise taxes significantly, or essential public services will collapse completely."
Why it's flawed: Other options exist: moderate tax increases, reducing spending in other areas, improving efficiency, seeking alternative revenue sources, or implementing public-private partnerships.
5. Circular Reasoning (Begging the Question)
The Flaw: The conclusion simply restates the premise in different words, providing no actual independent support. The argument assumes what it's trying to prove.
Example: "This policy is fair because it treats everyone justly."
Why it's flawed: "Fair" and "justly" mean essentially the same thing. The argument provides no independent reason why the policy is fair—it simply restates the claim.
6. Ad Hominem Attack
The Flaw: Attacking the person making an argument rather than addressing the argument's actual merits. The arguer's character is irrelevant to whether their reasoning is sound.
Example: "Senator Williams argues we should increase education funding, but she's been divorced three times, so her argument can't be trusted."
Why it's flawed: The senator's personal life has no bearing on the validity of her policy arguments about education funding. The argument attacks character instead of addressing substance.
7. Equivocation (Shifting Meaning)
The Flaw: Using a key term with one meaning in the premise and a different meaning in the conclusion. This creates the illusion of logical connection where none exists.
Example: "All laws should be just. But according to natural law, certain moral principles are laws. Therefore, certain moral principles should be just."
Why it's flawed: "Laws" means legal statutes in the first sentence but moral principles in the second. The shifting definition breaks the logical connection.
8. Appeal to Inappropriate Authority
The Flaw: Relying on an authority figure who lacks relevant expertise in the subject matter, or assuming expertise in one field translates to another.
Example: "Famous actor Smith says this investment strategy is sound, so it must be a good way to manage your retirement funds."
Why it's flawed: Being a successful actor doesn't provide expertise in finance or investment strategy. The authority cited is irrelevant to the conclusion.
Other Common LSAT Flaws
- Absence of Evidence ≠ Evidence of Absence: Concluding something doesn't exist just because it hasn't been proven to exist.
- Percent vs. Number Confusion: Confusing percentages with absolute numbers or rates with totals.
- Part-to-Whole Error: Assuming what's true of parts must be true of the whole (or vice versa).
- Temporal Reasoning Flaw: Assuming past or present trends will continue unchanged into the future.
- False Analogy: Drawing conclusions based on comparisons between fundamentally different situations.
- Straw Man: Misrepresenting someone's position to make it easier to attack.
- Confusing Likelihood with Certainty: Treating probable outcomes as if they were guaranteed.
The 6-Step Method for Solving Flaw Questions
Step 1: Identify the Conclusion
Find what the author is ultimately trying to prove. Look for conclusion indicators: "therefore," "thus," "hence," "so," "consequently," "it follows that," "we can conclude." The conclusion is the main claim that needs support.
Step 2: Identify the Premises
Determine what evidence, facts, or reasons support the conclusion. Premises are the building blocks used to construct the argument. Look for premise indicators: "because," "since," "given that," "for," "as indicated by."
Step 3: Find the Gap
Analyze the logical connection between premises and conclusion. Ask: "Does the conclusion NECESSARILY follow from the premises?" Identify unstated assumptions. What must be true for this reasoning to work? This gap is where the flaw lives.
Step 4: Recognize the Flaw Pattern
Match the gap to common flaw types: Is it correlation/causation? Necessary/sufficient confusion? Sampling error? False dichotomy? Recognizing patterns speeds up your analysis and improves accuracy.
Step 5: Predict the Answer
Before looking at answer choices, articulate the flaw in your own words. This prediction prevents you from being misled by cleverly worded wrong answers and helps you recognize the correct answer immediately.
Step 6: Evaluate Answer Choices
Compare each choice to your prediction. Ask two questions: (1) Does this answer describe something actually present in the argument? (2) Is what it describes actually a flaw? Eliminate answers that fail either test.
Worked Example 1: Correlation vs. Causation
Argument:
A recent study found that employees who work in offices with windows reported 15% higher job satisfaction than employees in windowless offices. Additionally, the employees in windowed offices demonstrated 12% higher productivity over a six-month period. Clearly, working near windows causes both increased job satisfaction and improved productivity. Therefore, companies should install windows in all office spaces to boost employee performance.
Question:
The reasoning in the argument is most vulnerable to criticism on the grounds that the argument:
Answer Choices:
(A) Fails to consider that employee satisfaction might not be the most important factor in determining productivity
(B) Overlooks the possibility that factors other than windows might account for the observed differences in satisfaction and productivity
(C) Assumes without justification that what improves satisfaction will necessarily improve productivity
(D) Takes for granted that the sample studied is representative of all office workers
(E) Neglects to specify the exact percentage increase necessary to justify the cost of window installation
Step-by-Step Solution:
1. Identify the Conclusion:
"Companies should install windows in all office spaces to boost employee performance" — This is what the author wants us to accept.
2. Identify the Premises:
• Employees in windowed offices: 15% higher satisfaction
• Employees in windowed offices: 12% higher productivity
• Windows "cause" these improvements
3. Find the Gap:
The argument observes a CORRELATION (windows present + higher satisfaction/productivity) and concludes CAUSATION (windows CAUSE the improvement). But correlation doesn't prove causation! Perhaps senior employees or higher-paid workers get windowed offices, and they're more satisfied/productive for reasons unrelated to windows (better pay, more interesting work, more experience).
4. Recognize the Flaw Pattern:
Classic Correlation vs. Causation flaw. The argument needs to eliminate alternative explanations.
5. Predict the Answer:
"The argument assumes that windows are the cause of the differences, but doesn't consider other factors that might explain why people in windowed offices perform better."
6. Evaluate Answer Choices:
(A) ❌ The argument doesn't claim satisfaction is the MOST important factor—just that windows help. This misses the causation flaw.
(B) ✅ CORRECT! This perfectly captures the causation flaw. Alternative factors could explain the correlation.
(C) ❌ The argument doesn't assume satisfaction leads to productivity—it claims windows independently improve both.
(D) ❌ While potentially a concern, this isn't the primary flaw. Even if representative, the causation problem remains.
(E) ❌ Cost-benefit analysis is out of scope. The argument's reasoning problem is about causation, not economics.
Correct Answer: (B)
The argument commits a correlation/causation fallacy by failing to consider alternative explanations for the observed differences between windowed and windowless offices.
Worked Example 2: Necessary vs. Sufficient Conditions
Argument:
Medical experts agree that regular exercise is essential for maintaining cardiovascular health. Since Maria exercises regularly by running three miles every morning, she will definitely maintain good cardiovascular health throughout her life.
Question:
The reasoning above is flawed because it:
Answer Choices:
(A) Relies on the opinion of medical experts without questioning their credentials
(B) Assumes that running is the only form of exercise that benefits cardiovascular health
(C) Treats a condition necessary for maintaining cardiovascular health as if it were sufficient
(D) Fails to define what constitutes "regular" exercise with adequate precision
(E) Confuses cardiovascular health with overall physical health
Step-by-Step Solution:
1. Identify the Conclusion:
"Maria will definitely maintain good cardiovascular health throughout her life" — This is the definitive claim being made.
2. Identify the Premises:
• Regular exercise is essential (necessary) for cardiovascular health
• Maria exercises regularly
3. Find the Gap:
The premise tells us exercise is NECESSARY (you must have it), but the conclusion treats it as SUFFICIENT (it alone guarantees the outcome). Just because you need exercise doesn't mean exercise alone is enough! Other factors matter: diet, genetics, stress management, avoiding smoking, regular checkups, etc.
4. Recognize the Flaw Pattern:
Confusing Necessary and Sufficient Conditions — specifically treating necessary as sufficient.
Logic breakdown:
Premise: Cardiovascular Health → Requires Exercise (If you have good CV health, you exercised)
Conclusion: Exercise → Cardiovascular Health (If you exercise, you'll have good CV health)
This reverses the logic incorrectly!
5. Predict the Answer:
"The argument treats something that's necessary for cardiovascular health (exercise) as if having it is enough to guarantee cardiovascular health."
6. Evaluate Answer Choices:
(A) ❌ The experts are appropriately qualified (medical experts on health). Their credentials aren't the issue.
(B) ❌ The argument doesn't claim running is the ONLY exercise—just that Maria exercises regularly.
(C) ✅ CORRECT! This precisely identifies the necessary/sufficient confusion. Exercise is necessary but not sufficient alone.
(D) ❌ The argument accepts that Maria's routine qualifies as "regular exercise." Defining it more precisely wouldn't fix the logical flaw.
(E) ❌ The argument specifically discusses cardiovascular health, not overall health. No confusion occurs here.
Correct Answer: (C)
The argument incorrectly assumes that because exercise is necessary for cardiovascular health, it must also be sufficient—ignoring other required factors like diet, genetics, and lifestyle choices.
Worked Example 3: Sampling Error / Overgeneralization
Argument:
A consumer advocacy group surveyed 50 customers who had recently purchased electric vehicles from premium dealerships in wealthy suburban neighborhoods. The survey found that 88% of respondents were "highly satisfied" with their purchases and would recommend electric vehicles to others. Based on these findings, the group concluded that electric vehicles are highly satisfactory for the general car-buying public and that most consumers would be pleased with electric vehicle ownership.
Question:
Which of the following most accurately describes a flaw in the argument?
Answer Choices:
(A) It fails to consider whether the 88% satisfaction rate is higher than satisfaction rates for gasoline-powered vehicles
(B) It draws a conclusion about all car buyers based on a sample that may not be representative of the general car-buying public
(C) It assumes that consumer satisfaction is the only relevant factor in evaluating vehicle quality
(D) It neglects to specify the exact features of the electric vehicles that customers found satisfactory
(E) It relies on self-reported satisfaction rather than objective performance metrics
Step-by-Step Solution:
1. Identify the Conclusion:
"Electric vehicles are highly satisfactory for the general car-buying public and most consumers would be pleased with electric vehicle ownership."
2. Identify the Premises:
• Survey of 50 customers from premium dealerships in wealthy suburbs
• 88% of this group highly satisfied
• 88% would recommend EVs to others
3. Find the Gap:
The argument leaps from a VERY SPECIFIC sample (wealthy suburban customers buying from premium dealerships) to a BROAD conclusion (the general car-buying public). This is a massive generalization! Wealthy suburban buyers likely have:
- Home charging capabilities (garages, driveways)
- Higher budgets for expensive vehicles
- Different commuting patterns
- Access to multiple vehicles
- Different priorities and needs than average buyers
4. Recognize the Flaw Pattern:
Sampling Error / Overgeneralization — Drawing broad conclusions from an unrepresentative, biased sample.
5. Predict the Answer:
"The argument generalizes from wealthy suburban buyers to all car buyers, but this sample isn't representative of the general public."
6. Evaluate Answer Choices:
(A) ❌ Comparing to gas vehicles is unnecessary for the argument's logic. The flaw is about the sample, not about comparative analysis.
(B) ✅ CORRECT! This precisely identifies the sampling problem—generalizing to "all car buyers" from a specific, unrepresentative group.
(C) ❌ The argument doesn't claim satisfaction is the ONLY factor—just that it's evidence of vehicle suitability.
(D) ❌ Specifying features wouldn't fix the sampling flaw. The problem isn't missing details, it's the unrepresentative sample.
(E) ❌ Self-reported satisfaction is appropriate for gauging consumer satisfaction. The flaw is who was surveyed, not how.
Correct Answer: (B)
The argument commits a sampling error by extrapolating from wealthy suburban premium dealership customers to the entire car-buying public—a clear overgeneralization from an unrepresentative sample.
Expert Tips for LSAT Flaw Questions
✓ Always Identify Conclusion First
The conclusion is your target. Everything else serves to support it (or fails to support it). Identifying it clearly prevents confusion when analyzing the argument's structure.
✓ Look for Assumption Gaps
Most flaws involve unstated assumptions that bridge premises to conclusion. Ask: "What must be true for this reasoning to work?" The gap usually reveals the flaw.
✓ Predict Before Reading Choices
Formulating your own answer prevents the LSAT from manipulating your thinking with attractive wrong answers. Strong predictions lead to faster, more confident selections.
✓ Eliminate Using Two Questions
(1) Is this thing actually in the argument? (2) Is it actually a flaw? An answer must pass both tests. Many wrong answers describe things that aren't present or aren't actually problems.
✓ Master Common Patterns
The same 10-15 flaw types appear repeatedly. Learning to recognize them instantly saves time and improves accuracy. Pattern recognition is key to LSAT success.
✓ Don't Add Information
Judge the argument only on what's stated. Don't bring in outside knowledge or make additional assumptions. The flaw must be identifiable from the text alone.
✓ Practice with Real LSAT Questions
Nothing substitutes for practicing with official LSAT PrepTests from LSAC. Third-party questions often don't capture the nuance and difficulty of real LSAT reasoning.
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Quick Reference: Flaw Question Stems
Recognize flaw questions by these common question stems:
- "Which one of the following most accurately describes a flaw in the argument's reasoning?"
- "The reasoning in the argument is most vulnerable to criticism on the grounds that the argument..."
- "The argument is questionable because it..."
- "A flaw in the argument is that the argument..."
- "The reasoning above is flawed in that it..."
- "The argument's reasoning is most vulnerable to the objection that..."
- "Which one of the following indicates an error in the reasoning leading to the prediction above?"
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This guide is designed to help you understand LSAT Logical Reasoning flaw questions. For the most current test information and official practice materials, always consult LSAC.org.