LSAT Flaw Questions

LSAT flaw questions test whether you can describe the reasoning error in an argument. The stimulus may sound confident and use persuasive facts, but the conclusion can go beyond what those facts prove. Your job is to characterize that gap accurately. A flaw answer describes the move from evidence to conclusion; it does not merely say that the conclusion might be wrong.

A three-part diagnosis

First identify the conclusion: what does the speaker want you to accept? Second identify the evidence: what facts are offered? Third compare their scope and relationship. Ask whether the author changes a condition's direction, turns correlation into causation, moves from a small sample to a population, ignores another explanation, or assumes that no evidence for a claim means the claim is false.

For example: “A town opened a public garden, and local restaurant sales increased. The garden caused the increase.” The evidence is two events occurring in sequence. The conclusion is causal. The argument does not rule out festivals, seasonal tourism, or other changes. Its flaw is treating a correlation or temporal sequence as enough to establish causation.

A concise description helps before you examine answer choices. “It assumes the garden caused the rise without ruling out other explanations” is more useful than “the argument is weak.” The second statement is true of many flawed arguments but does not identify this one.

Causal reasoning errors

A causal flaw appears when the argument claims that one factor produced an outcome based only on association, timing, or a before-and-after observation. Look for a possible common cause, reverse causation, an outcome that began before the alleged cause, or differences between the groups being compared.

Original example: “Employees who attend the optional leadership seminar receive higher performance ratings. The seminar raises performance, so every employee should attend.” Perhaps managers nominate employees already on track for promotion. Or high performers have more time to attend. The argument treats participation and ratings as causally linked without showing that the seminar produced the higher ratings.

Causal arguments often assume that a change in one factor is the only relevant difference. If a company introduces a training program at the same time it changes its evaluation system, later productivity scores cannot be attributed to training alone. A choice pointing out the second change may reveal the flaw even if training could still have helped.

Conditional reversals

A conditional statement says that one condition is sufficient for another, or that one outcome requires another condition. If A → B, you may infer B from A and not A from B. “Every accredited lab has a safety officer” means accredited → officer. It does not mean that every lab with an officer is accredited.

Original example: “All certified translators passed the language assessment. Mira passed the assessment. Therefore, Mira is a certified translator.” The argument reverses the conditional. The assessment could be passed by people who have not applied for certification or who have not met other requirements. A flaw answer should identify the assumption that satisfying a necessary condition is enough to establish the outcome.

A different conditional error denies the sufficient condition. From A → B, not A does not establish not B. If everyone admitted to a program has an interview, the absence of an interview shows non-admission; but not having an interview may mean the person is still waiting, not that they were rejected, unless the rule applies to all statuses and time points.

Generalization from a sample

A generalization flaw occurs when evidence from a subset is applied to a larger population without showing that the subset is representative. Ask who was surveyed, who responded, where the sample came from, how large it was, and whether it includes the groups the conclusion covers.

Example: “Most people who attended the downtown planning meeting support the new transit route. Therefore, most residents support it.” Meeting attendees may be unusually engaged or live near the proposed route. The evidence covers attendees, but the conclusion covers all residents. The sample-selection method is missing.

A small sample is not automatically defective; a carefully selected representative sample can be informative. The flaw is the unsupported leap from sample to population. Conversely, a large sample can still be biased if its members are selected in a way that excludes important groups or encourages self-selection.

Composition, division, and analogy

A composition error assumes that what is true of individual members is true of the group. Each musician in an ensemble may be an excellent soloist, but the ensemble may not perform well together. A division error moves the other way: a firm may be profitable overall, but that does not mean every department is profitable.

Analogical arguments infer that because two things are alike in some respects, they are alike in another. The flaw may be that the comparison ignores a relevant difference. If two towns both have heavy traffic but only one has frequent rail service, evidence that a parking tax worked in the first does not automatically show it will work in the second. The difference matters if it affects the policy's mechanism.

For analogy questions, do not require that the examples be identical. Ask whether the cited similarities relate to the conclusion and whether a disanalogy undermines that inference. A flaw answer may describe treating two situations alike despite a relevant difference, rather than simply saying the cases are not identical.

Missing evidence and false dilemmas

An argument from ignorance treats failure to prove a claim as proof that it is false, or treats lack of disproof as proof that it is true. “No study has shown the device is unsafe, so it must be safe” assumes that the available studies would have detected every relevant danger. Absence of evidence can matter, but only when the argument establishes that the evidence would likely be found if the claim were true.

A false dilemma presents two options as the only possibilities when other alternatives exist. “Either the school bans phones during lunch or students will never talk to each other” ignores intermediate policies and other social settings. The flaw is not necessarily that a ban would fail; it is that the argument treats a limited set of options as exhaustive without support.

A source attack may also be flawed when it rejects a claim solely because of who made it rather than addressing the evidence. If a report contains data and a speaker dismisses it only because the author works for a company, the reasoning may substitute a source's interest for an evaluation of the evidence. A conflict can be relevant, but it does not by itself show that the report is false.

Worked flaw example

Argument: “Every technician who completed the new safety course passed the equipment test. Jordan passed the equipment test. Therefore, Jordan completed the course.” The premise says course completion is sufficient for passing. The conclusion infers course completion from passing, which reverses the conditional. Jordan may have passed through prior experience or another course.

A good flaw description is: “It takes a condition that is sufficient for passing to be sufficient for course completion.” A poor description is: “It assumes Jordan is good at equipment.” That may explain the result, but it does not state the structural error. Another weak choice might say the argument assumes everyone completed the course; nothing in the premise says that.

Do not overstate the flaw

The answer should match the argument's actual weakness. If the stimulus gives a study without a control group, an answer saying that the study's conclusion is certainly false goes too far. The flaw is that the evidence does not establish causation. If the argument uses data from one city, an answer claiming the data are fabricated is unsupported unless the stimulus raises that issue.

Some arguments may have more than one weakness, but the credited answer must describe a flaw the reasoning actually commits. Do not select a general criticism simply because it could apply to the topic. A causal argument is not flawed merely because the sample is small if it makes no statistical generalization; a recommendation is not flawed merely because implementation costs are unknown if the premises do not rely on cost.

Why answer choices sound plausible

Distractors often describe a nearby but different error. One may identify a weakness in the evidence that is not used. Another may say the argument treats a sufficient condition as necessary when it actually makes the converse error. A third may criticize the conclusion's breadth when the argument only makes a narrow claim. Match the direction, quantifier, and scope precisely.

An answer can use impressive logical terminology and still be wrong. You do not need to memorize labels such as “affirming the consequent.” Translate the answer into ordinary words, then check whether the stimulus makes that move. Correct reasoning descriptions are often more precise than technical labels.

Mini practice: describe the flaw

Argument: “Every grant recipient submitted a project plan. The city received a project plan from this nonprofit, so the nonprofit will receive a grant.” The flaw is not that the nonprofit's plan is poor; the stimulus gives no information about its quality. The reasoning reverses recipient → plan into plan → recipient. Other organizations may submit plans without receiving grants.

A correct description would say that the argument treats a requirement for receiving a grant as sufficient to guarantee one. A choice saying the nonprofit's plan is incomplete is unsupported. A choice saying the city received no plans from other groups is not stated. A choice saying the nonprofit is not eligible confuses an alternative possibility with an established fact. The flaw is the conditional reversal, regardless of whether the nonprofit eventually receives the grant.

Now consider: “The town's new recycling signs appeared in April, and recycling volume rose in May. Therefore, the signs caused the increase.” This is a different flaw. The argument assumes causation from a sequence and does not consider other changes, such as a new collection schedule. A conditional-reversal answer would not describe this reasoning even though both arguments fail to prove their conclusions.

A repeatable process

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Review missed questions by reconstructing the argument from scratch. If you read a choice first and then persuade yourself it applies, you may be matching vocabulary rather than reasoning. Close the explanation, name the conclusion and support, state the gap, then reopen the choices. This practice makes the method transfer to new topics.

Sources

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