Logic Refined

Flaw practice

Find the gap in the argument.

A flaw question asks how the reasoning fails. The subject matter can change completely while the same mistake appears again.

Identify the conclusion before naming the flaw.

Start with the claim the author wants you to accept. Then identify the evidence offered for it. The issue is the step from that evidence to that conclusion, not whether the topic sounds doubtful or the conclusion could happen to be true.

Describe the mistake in ordinary language.

Before looking for a familiar label, explain what the author has taken for granted. Did a study test what the conclusion claims? Did a comparison leave out a relevant difference? Did the meaning of a term shift? A precise description is more useful than a broad label such as “bad evidence.”

Make every word of an answer earn its place.

An answer can describe a real reasoning mistake and still fail to describe this argument. Check the direction of the alleged inference, the strength of the conclusion and whether the author actually makes the assumption the answer attributes to them.

What to notice in the sample

The formula’s designer claims the new formula will forecast next season’s titles more accurately. The clerk checks the arithmetic on the same records used to build the formula. The question is whether an independent person has supplied independent evidence—not whether the arithmetic is correct.

A complete original example

Read it. Make a choice.
Then compare the reasoning.

This is the unchanged flaw example from the public sample set. These are original practice questions, not official LSAT questions.

A bookseller has two formulas for forecasting sales of new titles. The older formula uses a book’s genre and advance orders. The new formula also uses details of a book’s cover and description. Its designer chose the details and their weights after examining the recorded sales of every title released last spring, adjusting the formula to make its forecasts fit those records. An independent clerk then checked the arithmetic and confirmed that, on those same titles, the new formula had a lower average forecasting error than the old one. The designer argues: “The clerk’s independence answers the worry that I favor my own work. We have now demonstrated that, for next season’s new titles, the new formula will have a lower average forecasting error than the old one.”

The designer’s reasoning is most vulnerable to criticism on the grounds that it

  1. takes an association between features of books and their sales as establishing that those features cause people to buy the books.
  2. assumes that adding information to a forecasting method necessarily improves the accuracy of the forecasts it produces.
  3. treats independent checking of a calculation as making the observations used in that calculation an independent test of the formula.
  4. rejects a simpler forecasting method without establishing that complicated methods generally outperform simpler ones.
  5. infers that no particular forecast will be badly mistaken from a reduction in the average error of a group of forecasts.
Read the answer and all five explanations

Correct answer: C.

The clerk independently verifies the error calculation, but the formula was adjusted using the very sales being checked. Accurate arithmetic does not turn the records used to fit the formula into a separate test of its forecasts for new titles.

  1. A. takes an association between features of books and their sales as establishing that those features cause people to buy the books.

    The designer claims forecasting accuracy, not that covers or descriptions cause sales. Associations can help prediction without representing causal influences. The problem concerns how accuracy was tested, not a causal interpretation of the selected features.

  2. B. assumes that adding information to a forecasting method necessarily improves the accuracy of the forecasts it produces.

    The designer offers measured performance on last spring’s titles, not simply the presence of more information. That performance is inadequate as an independent test, but the argument does not claim that every additional detail must improve a forecast.

  3. C. treats independent checking of a calculation as making the observations used in that calculation an independent test of the formula.

    The clerk independently verifies the error calculation, but the formula was adjusted using the very sales being checked. Accurate arithmetic does not turn the records used to fit the formula into a separate test of its forecasts for new titles.

  4. D. rejects a simpler forecasting method without establishing that complicated methods generally outperform simpler ones.

    A particular method could outperform another without complicated methods being better in general. A general rule about complexity is unnecessary. What is missing is a test on titles that were not used to build the formula.

  5. E. infers that no particular forecast will be badly mistaken from a reduction in the average error of a group of forecasts.

    The comparison concerns average forecasting error, and the designer’s conclusion claims only a lower average error. It does not promise a small error for every title. Large individual mistakes can coexist with a lower average, and the designer never says otherwise.

See it for yourself

See the reasoning in action.

Read the arguments. Test your answer. Decide whether the explanations help you.

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