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Course: LSAT > Unit 1

Lesson 6: Logical Reasoning – Articles

Match a flaw | Learn more

How do we match a flaw in an argument to a flaw in a choice?

You can think of Match a Flaw questions as nearly identical to Match the Structure questions, but with the added information that the argument in the passage is flawed (and therefore, the argument in the answer will be as well). They can sometimes be long and/or time-consuming, so some students like to save them for last or skip them entirely.

What do Match a Flaw questions look like?

As with Match the Structure questions, you’ll see:

A flawed argument in the passage, and
Arguments in each of the choices—sometimes long ones!

The wording of the questions will be similar to the following:

Which one of the following arguments is most similar in its flawed reasoning to the argument above?

The pattern of questionable reasoning in the argument above is most similar to that in which one of the following?

Which one of the following arguments exhibits a flawed pattern of reasoning most similar to that exhibited by the argument above?

Example

Paleomycologists, scientists who study ancient forms of fungi, are invariably acquainted with the scholarly publications of all other paleomycologists. Professor Mansour is acquainted with the scholarly publications of Professor DeAngelis, who is a paleomycologist. Therefore, Professor Mansour must also be a paleomycologist.

The flawed pattern of reasoning in the argument above is most similar to that in which one of the following arguments?

(A) When a flight on Global Airlines is delayed, all connecting Global Airlines flights are also delayed so that the passengers can make their connections. Since Frieda's connecting flight on Global was delayed, her first flight must have also been a delayed Global Airlines flight.

(B) Any time that one of Global Airlines' local ticket agents misses a shift, the other agents on that shift need to work harder than usual. Since none of Global's local ticket agents missed a shift last week, the airline's local ticket agents did not have to work harder than usual last week.

(C) Any time the price of fuel decreases, Global Airlines' expenses decrease and its income is unaffected. The price of fuel decreased several times last year. Therefore, Global Airlines must have made a profit last year.

(D) All employees of Global Airlines can participate in its retirement plan after they have been with the company a year or more. Gavin has been with Global Airlines for three years. We can therefore be sure that he participates in Global's retirement plan.

(E) Whenever a competitor of Global Airlines reduces its fares, Global must follow suit or lose passengers. Global carried more passengers last year than it did the year before. Therefore, Global must have reduced its fares last year to match reductions in its competitors' fares.

How might we tackle this question?

✓ Identify a flaw in the passage’s argument.

If you can quickly spot a classic flaw, it’ll make it easier for you to find a matching choice, but that isn’t always possible. A great first step is to break the argument into its conclusion and evidence to help you spot flawed reasoning.

Conclusion

Professor Mansour must be a paleomycologist

because

Support

Paleomycologists are invariably acquainted with the scholarly publications of all other paleomycologists, [and]
Professor Mansour is acquainted with the scholarly publications of Professor DeAngelis, who is a paleomycologist.

✓ Diagram if needed.

In Match the Flaw questions, you can often diagram the passage’s argument, particularly if you see repeating terms that relate to each other. In our example, we do see such repeating terms, which we'll indicate with an initial:

X (Mansour) must be a Y (paleomycologist)

because

All Ys are Z (acquainted with the scholarship publications of all other paleomycologists) and
X is A (acquainted with the scholarly publications of one paleomycologist).

You may be able to spot the flaw from this! If you didn't, let's move to the next tip.

✓ Look for common flaw categories.

For details on flaws that you may see most often on the LSAT, you can consult one of our favorite articles as a reference: Types of Flaws.

We noticed the repetition of the terms “acquainted with the scholarly publications” and “is a paleomycologist.” We also saw that the evidence is all about the relationships of different terms: X, Y, Z, A. Finally, we see definite language such as “invariably”. All of these are clues that the argument is a conditional one. So, we should see if the arguer might be confusing necessary and sufficient conditions.

We cannot conclude that Professor Mansour must be a paleomycologist, just from Mansour being acquainted with the scholarly publications of one other paleomycologist. Furthermore, even if Mansour were acquainted with the scholarly publications of all other paleomycologists, that still wouldn’t be enough to conclude that Mansour is a paleomycologist. That’s because all paleomycologists know the work of all other paleomycologists, but that doesn’t exclude non-paleomycologists from also knowing the work of all paleomycologists!

Let's put this into simpler terms—and also stop talking about paleomycologists for a moment!

Here's another example: Just because all people who are accepted to law school have written a personal statement, we can't conclude that anyone who has written a personal statement will definitely get into law school!

So, this example about Professor Mansour does indeed exhibit a classic flaw of necessary/sufficient confusion.

Being acquainted with the scholarship is a necessary condition of being a paleomycologist, not a sufficient one. All paleomycologists are acquainted, but not everyone who’s acquainted is necessarily a paleomycologist.

So, our prediction can be:

The arguer takes something that’s supposed to be necessary and treats it like it’s sufficient. Lots of people could be acquainted with paleomycologists’ publications without being paleomycologists themselves.

Note: Not every flaw will fit into a category. If you encounter an argument that doesn’t feature any classic LSAT flaws, then characterize its logical gap in your own words. Try to use language that’s general enough that you can see how it might apply to a variety of arguments, and simple enough that you can really understand it.

✓ Eliminate/test the choices

When it’s time to analyze the choices, efficiency should be a top priority.

Pay extra attention to conclusions. If a choice’s conclusion is very different from the original argument’s conclusion, it’s not likely to have the same flaw.
Look for clues. If you’re looking for a flaw that has clues associated with it (such as a causal relationship), scan for those typical terms and structures.

More info on flaw types

(A) When a flight on Global Airlines is delayed, all connecting Global Airlines flights are also delayed so that the passengers can make their connections. Since Frieda's connecting flight on Global was delayed, her first flight must have also been a delayed Global Airlines flight.

This choice’s flaw matches the passage’s flaw. The structure here is:

Conclusion

Frieda’s first flight must have been a delayed Global Airlines flight

because

Support

When a flight on Global Airlines is delayed, all connecting Global Airlines flights are also delayed, [and]
Frieda’s connecting flight on Global was delayed.

The problem here is the same as what we saw in the passage. The arguer cites Frieda’s one delayed connecting flight on Global Airlines as proof that Frieda’s first flight must have been a delayed Global Airlines flight. But the support is that when a Global flight is delayed all connecting Global flights are delayed. So the logic is reversed and we see the same shift in scope from all to one.

(B) Any time that one of Global Airlines' local ticket agents misses a shift, the other agents on that shift need to work harder than usual. Since none of Global's local ticket agents missed a shift last week, the airline's local ticket agents did not have to work harder than usual last week.

This choice is flawed, but its flaw doesn’t match the passage’s flaw. The passage mixes up a necessary condition with a sufficient condition (in other words, reverses the logic improperly), but this choice commits a different flaw:

if an agent misses a shift $\to$ ‍ other shift agents need to work harder than usual
So, since no agents missed a shift last week $\to$ ‍ the other agents did not have to work harder than usual.

This is analogous to arguing, “Any time it snows, my mother gets sad. Since it didn’t snow last week, my mother must not have been sad.” It’s flawed because the speaker’s mother could have gotten sad for some reason other than the snow, but that’s not a match for what we see in the passage. Additionally, we don’t see the same shift from all to one in this choice’s argument.

(C) Any time the price of fuel decreases, Global Airlines' expenses decrease and its income is unaffected. The price of fuel decreased several times last year. Therefore, Global Airlines must have made a profit last year.

This choice doesn’t confuse necessary and sufficient conditions. Given that the price of fuel decreased several times last year, we can deduce that the airline’s expenses decreased and its income was unaffected. The problem is that we’re missing information about whether there was an actual profit made, and that’s not the same problem that the passage exhibits.

(D) All employees of Global Airlines can participate in its retirement plan after they have been with the company a year or more. Gavin has been with Global Airlines for three years. We can therefore be sure that he participates in Global's retirement plan.

This argument overstates its conclusion. The support establishes that Gavin can participate in the plan, but not that he actually does. This is flawed reasoning, but isn’t the flaw we’re seeking..

(E) Whenever a competitor of Global Airlines reduces its fares, Global must follow suit or lose passengers. Global carried more passengers last year than it did the year before. Therefore, Global must have reduced its fares last year to match reductions in its competitors' fares.

The support says that passengers went up overall. The argument implies that the company must not have let its competitor undercut its prices, because that would have depressed the number of passengers. But again, the logic confuses relative claims and absolute claims. There are many other factors affecting how many passengers Global Airlines gets. Just because the absolute number of passengers went up doesn’t mean that something didn’t cause there to be fewer passengers than there would have been otherwise. The key distinction is between “few” and “fewer.”

Summary

✔ Identify the flaw in the passage’s argument
✔ Diagram if needed
✔ Look for common flaw categories
✔ Eliminate/test choices

Common Incorrect Choices

Some incorrect choices for Match a Flaw questions present arguments that aren’t flawed at all. Most wrong choices do present flawed arguments, but the reasoning errors in those arguments are different from the one in the reference argument.

Your turn

Match a flaw 1

Inspector: The only fingerprints on the premises are those of the owner, Mr. Tannisch. Therefore, whoever now has his guest's missing diamonds must have worn gloves.

Which one of the following exhibits a flaw in its reasoning most similar to that in the inspector's reasoning?

(Choice A)
The campers at Big Lake Camp, all of whom became ill this afternoon, have eaten food only from the camp cafeteria. Therefore, the cause of the illness must not have been something they ate.
(Choice B)
The second prototype did not perform as well in inclement weather as did the first prototype. Hence, the production of the second prototype might have deviated from the design followed for the first.
(Choice C)
Each of the swimmers at this meet more often loses than wins. Therefore, it is unlikely that any of them will win.
(Choice D)
All of Marjorie's cavities are on the left side of her mouth. Hence, she must chew more on the left side than on the right.
(Choice E)
All of these tomato plants are twice as big as they were last year. So if we grow peas, they will probably be twice as big as last year's peas.

Match a flaw 2

Policy analyst: Increasing the size of a police force is only a stopgap method of crime prevention; it does not get at the root causes of crime. Therefore, city officials should not respond to rising crime rates by increasing the size of their city's police force.

The flawed reasoning in which one of the following arguments most closely resembles the flawed reasoning in the policy analyst's argument?

(Choice A)
Some people think that rules with higher standards than people can live up to, such as those enjoining total honesty, prevent some immoral behavior by giving people a guide to self-improvement. But such rules actually worsen behavior by making people cynical about rules. Thus, societies should not institute overly demanding rules.
(Choice B)
Swamps play an important role in allaying the harsh effects of floods because they absorb a great deal of water. Although dams prevent many floods, they worsen the effects of the greatest floods by drying up swamps. Thus dams should not be built.
(Choice C)
Although less effective in preventing theft than security guards, burglar alarm systems are more affordable to maintain. Because the greater loss from theft when alarms are used is outweighed by their lower cost, companies are advised always to use burglar alarm systems.
(Choice D)
Because taking this drug does not cure the disease for which it is prescribed, but only reduces the disease's most harmful effects, doctors should not continue to prescribe this drug.
(Choice E)
We will never fully understand what causes people to engage in criminal activity. Therefore, we should investigate other ways to improve society's ability to combat crime.

The policy analyst concludes that a certain action (increasing the size of the police force) shouldn’t be taken simply because it doesn’t fix the entire problem (crime). However, that action does have some effect on crime, so it’s not necessarily reasonable to conclude that the city’s police force shouldn’t increase in size at all. After all, isn’t some help better than no help?

Which choice exhibits a similar error in reasoning?

Some final notes about matching flaws

Practice Identify a Flaw questions first: If you struggle with Match a Flaw questions, try practicing Identify a Flaw questions until can more easily recognize the logical gaps in LSAT arguments.
Consider skipping Many students find that Match a Flaw questions take longer to tackle than other question types. Because of that, many students strategically skip Match a Flaw questions entirely, or leave them until last. Remember never to leave questions blank though – there’s no penalty for guessing!
You can write on your test booklet! Don’t be afraid to diagram directly on top of the passage in your test booklet. Some students like to draw an “X” right on top of the repeating terms and a “Y” on top of a different set of repeating terms (for example).

Want to join the conversation?

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Liza W
Posted a year ago. Direct link to Liza W's post “Since the LSAT is now off...”
Since the LSAT is now offered digitally, and there is often not a physical text booklet, the notes may need to be adjusted to reflect the current way that the test is administered. I noticed this in several other sections as well.
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(22 votes)
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Jaewon Lim
Posted 2 years ago. Direct link to Jaewon Lim's post “A journey of a thousand m...”
A journey of a thousand miles begins with a single step, ^^
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(14 votes)
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Austin Lansdell
Posted 5 years ago. Direct link to Austin Lansdell's post “I'm confused as to why an...”
I'm confused as to why answer D is incorrect for the Paleomycologists question: the explanation provided states that "The support establishes that Gavin can participate in the plan, but not that he actually does." If the flaw is confusing a necessary assumption for a sufficient assumption, doesn't this explanation prove rather than disprove that example D actually fits that flaw?
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- Simon Rakei
  Posted 9 months ago. Direct link to Simon Rakei's post “"The support establishes ...”
  "The support establishes that Gavin can participate in the plan, but not that he actually does," means, as I read it, that Gavin satisfies the right hand criteria for the necessary condition to be met in order for him to be eligible to participate in the retirement plan. However, the choice doesn't necessarily imply that it is taking the Y right hand condition (working for three years) to then infer "if Y, then X." In other words, the choice doesn't imply a mistaking of a necessary condition for a sufficient condition. Rather, the structure of the argument itself in D is consistent with the original conditional of the stimulus which follows the logic of if X, then Y.
  
  The passage in D tells us:
  
  All (sufficient trigger) employees of Global Airlines can participate in its retirement plan (the goal or conclusion X)
  after (necessary condition) they have been with the company a year or more. (this is the necessary condition Y)
  So we can diagram this info as X >>> Y
  
  The structure of the argument in D is
  Conclusion:
  
  We can therefore be sure that he [Gavin] participates in Global's retirement plan.
  
  because
  
  Support:
  
  [All employees of Global Airlines can participate in its retirement plan (X) after they have been with the company a year or more (Y)]. And
  Gavin has been with Global Airlines for three years. (Y)
  
  Therefore, the argument concludes, we can be sure that Gavin participates in the retirement program. The chain of logic here is not reversed. It is almost circular in a way because it restates the opening premise by substituting Gavin after he meets the requirement. Therefore the diagram for the argument in choice D is X >>> Y.
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prestinleblanc12
Posted a year ago. Direct link to prestinleblanc12's post “The second practice quest...”
The second practice question about increased police force should be removed and replaced. We know for a fact that increased sizes of police forces in reality do nothing for rising crime rates and oftentimes create more state-sanctioned violence in communities that may be predisposed to crime.
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