This logical reasoning LSAT example shows exactly how to solve a flaw question. You will identify the conclusion, separate it from the evidence, describe the reasoning gap, and eliminate four wrong answers. The question is original, but the method applies directly to LSAT Logical Reasoning.
The example question
> Argument: > At the beginning of this month, the city library extended its Saturday hours. This month, more unique city residents visited the library on Saturdays than did last month. Therefore, the extended Saturday hours caused an increase in the number of unique city residents who visited the library on Saturdays. > > Question: > The argument is most vulnerable to criticism on the grounds that it > > A. treats evidence that a policy had an effect as evidence that the policy was justified > B. fails to consider that a change other than extended Saturday hours may have caused the increase in unique Saturday visitors > C. fails to distinguish between city residents who visit the library and city residents who borrow materials > D. bases a general claim about libraries on evidence from only one library > E. overlooks the possibility that some city residents dislike extended Saturday hours
The correct answer is B.
The key is not merely to think, “correlation does not prove causation.” That is a useful label, but the LSAT wants you to identify the precise unsupported move in this particular argument.
Step 1: Find the conclusion
The conclusion is the claim the author is trying to establish:
> The extended Saturday hours caused an increase in the number of unique city residents who visited the library on Saturdays.
Ask yourself: “If someone challenged this argument, what claim would the author defend?” That is usually the conclusion.
This conclusion is causal. It says one event, extending Saturday hours, produced another event, an increase in unique Saturday visitors. Causal conclusions should make you alert because evidence of a change followed by a result may not establish what caused that result.
Step 2: Separate the evidence from the conclusion
The evidence consists of two facts:
- At the beginning of this month, the library extended its Saturday hours.
- More unique city residents visited on Saturdays this month than last month.
These facts provide a clear comparison: last month was before the extension, and this month was after it. But they still do not establish why the count rose.
Perhaps a nearby community event brought more people to the area. Perhaps a school assignment encouraged students to use the library. Perhaps the library changed its programming or advertised Saturday services more heavily. You do not need to prove that any alternative explanation occurred. You only need to notice that the argument did not address alternatives.
Step 3: State the flaw in plain English
The flaw is an unsupported causal inference:
> The argument treats an increase that followed the schedule change as proof that the schedule change caused the increase.
A more precise version is:
> The argument fails to consider or rule out another relevant change that could explain the higher number of unique Saturday visitors.
That is why answer choice B is correct. It does not claim that another cause definitely existed. It says the author failed to consider a possibility that would weaken the causal conclusion.
Step 4: Predict the answer before reading the choices
Before looking closely at the answer choices, write a short prediction:
> “It assumes the longer hours caused the increase instead of considering other explanations.”
This habit matters. Answer choices are designed to sound plausible, especially when they use familiar LSAT terms. A prediction gives you a standard: the correct answer must describe the logical gap you already found.
Your prediction does not need formal test-prep language. Plain English is often better because it keeps you focused on the argument rather than on vocabulary.
Step 5: Eliminate the wrong answers
A. Treats an effect as evidence that the policy was justified
This answer shifts the issue to whether extending hours was a good decision. The argument does not claim that the policy was wise, efficient, or justified. It claims that the policy caused more unique city residents to visit on Saturdays.
A policy can cause an effect without being justified overall. Eliminate A.
B. Fails to consider another possible cause
This is the gap. The argument observes a higher visitor count after the schedule change and concludes that the schedule change caused it. It never considers whether another relevant change could explain the increase.
Keep B.
C. Fails to distinguish visitors from borrowers
The argument measures unique city residents who visited the library. It makes no claim about borrowing materials. Whether some visitors borrowed books is irrelevant to the causal inference.
This is a common wrong-answer pattern: it introduces a detail related to the setting but unrelated to the reasoning. Eliminate C.
D. Generalizes from one library to all libraries
The conclusion concerns this city library and its Saturday visitors. It does not make a claim about libraries generally.
When an answer choice alleges overgeneralization, compare its scope with the conclusion's scope. If the conclusion remains narrow, the criticism does not fit. Eliminate D.
E. Overlooks people who dislike longer hours
Some residents might dislike the change. That does not challenge the claim that longer hours caused an overall increase in unique Saturday visitors.
This answer raises a possible downside, not an alternative explanation for the evidence. Eliminate E.
A repeatable method for LSAT flaw questions
Use this sequence for any Logical Reasoning flaw question.
1. Mark the conclusion
Look for words such as “therefore,” “thus,” “so,” and “hence.” Then confirm the statement's role: it should be the claim supported by the other statements.
2. Label the evidence
Summarize each premise in a few words. In this example, the evidence was “hours changed this month” and “unique Saturday visitors increased.”
3. Identify the jump
Ask: “What would have to be true for these premises to establish that conclusion?”
For causal arguments, frequent gaps include:
- Another cause may explain the result.
- The supposed cause may not have occurred before the effect.
- The cause may have occurred alongside another important change.
In the library argument, the timing supports the claim that the extension came before the increase. The remaining gap is that another change could also account for the increase.
4. Predict the flaw
Write one sentence before evaluating the choices. Here: “The argument fails to rule out another cause of the increase.”
5. Check scope and relevance
The correct answer describes the reasoning actually used. Reject choices that criticize a claim the author never made, introduce a new issue, or use broader language than the conclusion supports.
A second mini-example: unsupported comparison between groups
Not every flaw question involves causation. Consider this original argument:
> Every member of College A's debate team completed one practice set this week. Therefore, College A's debate team completed more practice sets this week than College B's debate team.
The conclusion compares the total work of two teams. But the evidence gives information only about College A's members. It tells us nothing about College B's team size or how many practice sets its members completed.
A strong flaw description would be:
> The argument draws a comparison between two groups without providing enough information about one of those groups.
The topic is debate practice, but the reasoning pattern is an unsupported comparison. On the LSAT, name the inference the author made, not the subject matter of the stimulus.
Copy-paste practice template
After completing a flaw question, use this template in your notes:
``text
Conclusion:
Evidence:
Type of reasoning:
What the evidence does not establish:
My predicted flaw:
Why the correct answer matches:
Why each tempting wrong answer fails:
``
For timed practice, reduce it to this:
``text
C = ?
E = ?
Gap = ?
``
Start by writing full explanations. Once you recognize recurring patterns, the shorter version helps you work faster without losing accuracy.
For more repetition, visit PurrLearn and choose a logical-reasoning practice activity. When you practice causal arguments, focus on separating the timing of events from evidence that one event actually produced the other.
FAQ
What is a flaw question on the LSAT?
A flaw question asks you to identify an error in an argument's reasoning. The correct answer describes the gap between the premises and conclusion; it does not simply disagree with the conclusion.
How do I find the conclusion in LSAT Logical Reasoning?
Ask which statement the author is trying to prove. The other statements are generally evidence offered in support. Indicator words can help, but the logical role of each statement matters more.
Is “correlation is not causation” enough for an LSAT flaw question?
It is a useful starting point, but it is usually too broad. State the specific missing possibility, such as the argument's failure to consider another cause of the observed result.
Why are wrong answers tempting in flaw questions?
Wrong answers often mention a real detail from the stimulus or use familiar logical language. They are still wrong if they do not describe the argument's actual reasoning error.
Should I predict the flaw before reading the answer choices?
Yes. A brief prediction keeps answer choices from directing your analysis. You can then choose the option that most accurately matches the gap you identified.
Conclusion
In this logical reasoning LSAT example, the evidence showed that unique Saturday visitors increased after the library extended its hours. The conclusion went further by claiming that the extended hours caused the increase without addressing other possible explanations. Find that extra step, describe it precisely, and flaw questions become far more manageable.