LSAT Flaw Questions: The Recurring Flaw Types, Named and Explained
The LSAT reuses a small set of reasoning flaws. Learn to name them — causal, sampling, percent-vs-number, equivocation, circular, ad hominem — so flaw questions become recognition, not analysis.
2026-06-01 · 9 min read
Why naming the flaw is the whole game
Flaw questions ask you to describe the error in an argument's reasoning. The LSAT recycles the same flaws over and over, dressed in new topics. If you can name the flaw before reading the answers, you stop falling for choices that describe a real flaw — just not the one in this argument.
Treat this as a catalogue to memorize. Once these become familiar, most flaw answers read like multiple choice for a flaw you have already spotted.
The causal flaws
Correlation-to-causation: two things happen together, so the author claims one caused the other. The flaw ignores coincidence, reverse causation, and a third common cause.
Post hoc: B followed A, so A caused B. Sequence is not causation.
The numbers and sampling flaws
Unrepresentative sample: a conclusion about a whole group based on a sample that is too small, self-selected, or biased.
Percent vs. number: confusing a change in percentage with a change in raw count. A larger percentage of a smaller total can be fewer people.
The language and structure flaws
Equivocation: a key term shifts meaning between premise and conclusion.
Circular reasoning: the conclusion is assumed in the premises rather than supported by them.
Necessary-vs-sufficient confusion: treating a required condition as if it were enough, or vice versa.
The relevance flaws
Ad hominem / source: attacking the person or source instead of the argument.
Appeal to authority: treating an expert's say-so as proof, especially outside their field.
Straw man: distorting an opponent's claim and then refuting the distortion.
Once you can sort an argument into one of these buckets, the right answer is the choice that abstractly describes that bucket — and the traps are the choices describing the other buckets.
Frequently asked questions
How many flaw types do I actually need to know for the LSAT?
A working set of roughly ten to twelve recurring flaws covers the large majority of flaw questions: causal, post hoc, sampling, percent-vs-number, equivocation, circular, necessary/sufficient, ad hominem, appeal to authority, straw man, and false dilemma.
Should I name the flaw before or after reading the answers?
Before. Predicting the flaw in your own words first makes you far less likely to be tempted by answers that describe a real but irrelevant error.
Are flaw answers worded abstractly on purpose?
Yes. Correct flaw answers are usually written in general terms ("takes a sufficient condition to be necessary") rather than referencing the specific topic, which is why recognizing the abstract pattern matters.
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