At the end in the "Some final thoughts on Necessary Assumptions" Lean towards the choice with the weaker language—necessary assumptions are more often characterized by weaker statements than sufficient assumptions are. Can anyone explain why, what the logic would be for this?

Imagine an argument like the one below. Katy: “I know it’s going to rain tomorrow in the town between these two times: 1 o’clock and 2 o’clock. I know it’s going to rain the day after tomorrow in the town between these two times: 3 o’clock and 4 o’clock. *I cannot go with you, Mary, to the plaza or the park this week*. I don’t have an umbrella, and I don’t have a coat; I don’t have anything that could protect me from these weather events. I know you planned these places in advanced for me, and I am sorry.” There are three necessary assumptions: 1) Katy does not have any free time besides the time when it's going to rain on those particular days. 2) Katy cannot, for some reason, share an umbrella with Mary. 3) It's not possible for Mary to rearrange the times to meet with Katy. Looking at #1, you can make the necessary assumption too strong by assuming that Katy does not have _any_ free time. However, we are only trying to bridge the gap between her main conclusion and her supporting evidence in order for it to work. If the answer were too strong, then you would be *guaranteed* the argument works _rather than_ the argument *must need the assumption* to work. If Katy cannot go to those places in that week because of the weather events, and Katy argues she cannot protect herself from those weather events, then it is necessary to assume Katy does not have any free time besides the time when it's going to rain on those particular days, which is why Mary chose those particular times in the first place. Remember: necessary assumptions *must be true* for an argument to work. Sufficient assumptions *guarantees* the argument works.

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Course: LSAT > Unit 1

Lesson 6: Logical Reasoning – Articles

Necessary assumptions | Learn more

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How do we identify a necessary assumption?

Necessary assumption questions ask you to determine what has to be true—but was never explicitly stated—in order for an argument to work. Stated differently, if a necessary assumption were discovered to be false, the argument would fall apart—the conclusion would not follow logically from its premises.

How do we know that fuel is necessary for a rocket to be launched? Because if the rocket didn't have fuel, it couldn't be launched.

Therefore, if we conclude that the rocket will be launched on schedule, one "necessary assumption" would be that the rocket will be properly fueled in time—even if that is not stated as one of the premises for the argument.

What do necessary assumption questions look like?

Necessary assumption questions ask you to identify a choice that has to be true in order for the reasoning of the argument to follow logically:

The argument relies on assuming which one of the following?

The argument depends on the assumption that

Which one of the following is an assumption required by the argument?

Mind the gap

Necessary assumption questions usually involve arguments that have something—or several things—missing. There's at least one significant gap.

When there’s a gap in an argument (in other words, a missing link between support and conclusion) that argument is assuming that something is true without stating it. In other words, it’s an assumption.

Let's look at an example together.

Example

Educator: Reducing class sizes in our school district would require hiring more teachers. However, there is already a shortage of qualified teachers in the region. Although students receive more individualized instruction when classes are smaller, education suffers when teachers are underqualified. Therefore, reducing class sizes in our district would probably not improve overall student achievement.

Which one of the following is an assumption required by the educator's argument?

(A) Class sizes in the school district should be reduced only if doing so would improve overall student achievement.

(B) At least some qualified teachers in the school district would be able to improve the overall achievement of students in their classes if class sizes were reduced.

(D) Hiring more teachers would not improve the achievement of any students in the school district if most or all of the teachers hired were underqualified.

(E) Qualified teachers could not be persuaded to relocate in significant numbers to the educator's region to take teaching jobs.

✓ Identify the conclusion and support.

As you approach many types of Logical Reasoning questions on the LSAT, you'll set yourself up for success if you separate the conclusion from the support. This is very helpful for Necessary Assumption questions.

Conclusion: Reducing class sizes in our district would probably not improve overall student achievement

because

Support

Reducing class sizes in our school district would require hiring more teachers.
However, there is already a shortage of qualified teachers in the region.
Students receive more individualized instruction when classes are smaller
But education suffers when teachers are underqualified.

✓ Find a gap or overlooked possibility, if there is one.

Our next step is to figure out what’s missing. There's likely a missing link between the support and conclusion—or there might be several. If you don’t see any, try attempting to rebut the argument to find possibilities that the argument might have overlooked—that may allow you to detect the gap in the argument.

The conclusion in our example states that reducing class sizes probably won’t produce a net benefit. The evidence identifies a benefit and a drawback:

Benefit: Smaller classes provide more individualized instruction.

Drawback: Smaller classes mean more, and therefore less qualified, teachers.

Let's suppose we can't see the gap right away. We can investigate any gaps by trying to attack the argument:

Why do the teachers have to be less qualified, just because there's a shortage of qualified teachers in the region? Can't the district find qualified teachers elsewhere?
If the teachers do have to be less qualified, what about other benefits to smaller classes? Isn't it possible that the benefits of smaller classes outweigh the drawbacks of less qualified teachers?

Since we came up with two ways to attack the argument, the argument—in order to work—must be assuming:

The district can't find qualified teachers elsewhere.
The benefits of smaller classes don't outweigh the drawbacks of less qualified teachers.

It's possible that there are additional necessary assumptions to this argument, but these are the two that seem to stand out the most quickly.

Top tip: Overlooked possibilities are just the opposite of a necessary assumption. If you determine that the arguer is overlooking the possibility that the district could find qualified teachers outside the region, then the corresponding necessary assumption is that the district can't find qualified teachers outside the region. Because if we learned that the district could bring in qualified teachers from outside the region, the argument would fall apart!

✓ Match a choice to your prediction if possible. Otherwise, test each choice.

If you've made a strong prediction, you may be able to scan and locate the match for it very quickly. That's the best-case scenario!

If you couldn't make a strong prediction, you can evaluate each choice by asking if that choice's claim has to be true in order for the argument to hold together.

It’s great if you can make a prediction after you analyze the stimulus, since it’s easy to get lost in the choices if you have no idea what you're looking for. You won’t always be able to make a strong prediction, but it’s a good idea to practice predicting until it becomes easier.

Try evaluating each of our example's choices, and then check your evaluation:

(A) Class sizes in the school district should be reduced only if doing so would improve overall student achievement.

This statement doesn't match our prediction, and it isn't necessary to the argument. The educator draws a conclusion about whether reducing class sizes would improve student achievement. This choice, on the other hand, is a recommendation about when you'd want to reduce class sizes.

You can also think of this choice as being wrong because it doesn't have to be true that there's only one condition (improving achievement) under which it's appropriate to reduce class sizes. That's too strong of a statement, since it's within the allowances of the argument that there are many situations in which class sizes should be reduced.

(B) At least some qualified teachers in the school district would be able to improve the overall achievement of students in their classes if class sizes were reduced.

If anything, this could weaken the argument, depending on how much impact these qualified teachers would have after class sizes were reduced. If these qualified teachers improved the overall student achievement enough, then the conclusion would be false (that an overall improvement in student achievement can't be attained).

(C) Students place a greater value on having qualified teachers than on having smaller classes.

This comparison isn't relevant to the argument. What students place value on isn't explicitly linked to whether student achievement would improve if class sizes were reduced. And even if we made the very tenuous connection that what students value affects how much they can achieve—and we don't want to speculate this way on Test Day—it still isn't a necessary assumption. It's possible that the value students place on having qualified teachers and having smaller classes are both very high—with one being just slightly higher than the other.

(D) Hiring more teachers would not improve the achievement of any students in the school district if most or all of the teachers hired were underqualified.

This choice is too strong to be necessary to the argument. It essentially states that no students would improve their achievement if most of the additional teachers hired were underqualified. However, the educator is making an argument about overall student achievement—not the achievement of individual students.

(E) Qualified teachers could not be persuaded to relocate in significant numbers to the educator's region to take teaching jobs.

This is a necessary assumption of the educator's argument! For the educator to rule out the possibility of increased student achievement due to the shortage of qualified teachers in the region, one thing that educator must be assuming is that there isn't a way to access qualified teachers outside of the region.

You can also think of it this way: if qualified teachers could be persuaded to relocate to the region to take teaching jobs, the argument would fall apart! So, it's a necessary assumption that qualified teachers can't be persuaded to relocate to the region to teach.

✓ Test choices by negating.

This strategy is somewhat time-consuming, and you definitely don't have time to test each choice in this way, so it tends to be most helpful for situations in which:

You're fairly certain you have the answer but you want to double-check, and you have the time to do so.
You've eliminated down to two choices and are feeling stuck, and you have the time to test them by negating.

How it works: A necessary assumption has to be true in order for an argument to work. Therefore, an argument will be be severely undermined if a necessary assumption is deemed to be false. This points to a useful test:

To determine whether a choice is a necessary assumption, suppose that the choice were false:

If the argument falls apart when you do so—that is, the conclusion no longer follows logically from the support—then that choice is the answer.
If the argument is unaffected or strengthened, then the choice isn't a necessary assumption.

Why it works: If you take away something that's necessary, then you encounter failure.

A personal statement is necessary to complete a law school application. If you take away the personal statement, then the law school application fails to be completed.
Sunlight is necessary for Plant X to survive. So if you take away the sunlight, Plant X will die.

On the flip side, if you take away something that's not necessary, then you won't encounter automatic failure.

Prada shoes are not necessary to my survival. So if you take my Prada shoes away, I might be sad, but I wouldn't die.
Water is necessary to survival, but triple-filtered water from the springs in a tucked-away Himalayan mountain is not necessary to survival. So if you take away that fancy water, I won't die (as long as you give me some kind of less-fancy water to replace it).

In our example question, the argument was essentially that reducing class sizes won't improve student achievement, because it would require more teachers, and there's already a shortage of qualified teachers in the region.

Let's look at just three of the choices to demonstrate the negation test:

(B) At least some qualified teachers in the school district would be able to improve the overall achievement of students in their classes if class sizes were reduced.

Negation: No qualified teachers in the school district would be able to improve the overall achievement of students in their classes if class sizes were reduced.

Result: This would actually support the argument, which is skeptical about how beneficial smaller classes would be. That means that in its original form, this choice weakens the argument, which is the opposite of being a necessary assumption.

(C) Students place

on having qualified teachers than on having smaller classes.

Negation: Students don’t place a greater value on having qualified teachers than on having smaller classes.

Result: This would not affect the argument, which doesn't involve students’ value judgments. Since the argument is unaffected when we negate this statement, then it must not be necessary in its original form.

(E) Qualified teachers could not be persuaded to relocate in significant numbers to the educator's region to take teaching jobs.

Negation: Qualified teachers could be persuaded to relocate in significant numbers to the educator’s region to take teaching jobs.

Result: This seriously weakens the argument. When we negate (E), qualified teachers can simply be recruited from other areas. Because it makes irrelevant the support that the conclusion was balancing on, negating this assumption makes the argument fall apart. Therefore, in its original form this choice is necessary to the argument.

Sometimes it's straightforward to negate. Other times, negating can get complicated—especially when quantifiers like "some," "all," "few," "most" or "none" are involved.

Top tip: It can often be helpful to use the phrase “It's not the case that ________.” You can add it to beginning of the statement to simply and accurately negate it.

Take a sentence like, “All lawyers must study contracts.” What is its negation?

"No lawyers must study contracts?"—No, because the negation of "all" is not "none." It's simply "not all", which could be 99%, 1%, 0%, etc.
"Not all lawyers must study contracts?"—Yes, but this risks sounding unnecessarily complex and wordy.
"All lawyers must not study contracts?"—No. This means that 100% of lawyers need to abstain from studying contracts; this isn't a negation of the original statement.

You can avoid having to think about it. A less stressful way to negate the statement is simply:

“It's not the case that all lawyers must study contracts.”

Here are some negations that many students find helpful. The negation works in both directions, not just from left to right:

Term	Negation	Note
All	Not all	"Not all" could mean none, but it could also mean some, or even a lot.
None	Some	"Some" means at least one, but it extends all the way to "all"!
Hot	Not hot	If something isn't hot, that doesn't mean that it's cold. It could be lukewarm, frozen, room temperature, etc.
Rarely	Not rarely	"Not rarely" could indicate that it's all the time, or maybe half the time—it's just not rarely.
Most	Not most	This one can be challenging for students. "Not most" means "not more than 50%." It could be exactly 50%, or it could be less, and "not most" also includes the possibility of 0%.
More than	Less than or equal to	Students sometimes overlook the possibility that one value is equal to the other, if it's not more than the other.

Summary

✓ Identify the conclusion and support.
✓ Find a gap or overlooked possibility, if there is one.
✓ Match a choice to your prediction or eliminate strategically.
✓ Negate a choice if necessary.

Common wrong choice types

Too strong: When a choice is too strong, you’ll want to ask , “Does this statement really have to be true, to the degree that’s expressed here, in order for the argument to hold? Or is it going too far in its precision or strength?” Example: If your prediction is, "Some of the diseases must be curable", a choice that's too strong might state, "Most of the diseases must be curable."
Irrelevant: When a choice is irrelevant to the argument, you’ll want to ask yourself, “Does this even matter to the argument, or is it just something that’s ‘nice to know’ or ‘surprising because it’s not mentioned or important to the argument'?"
Weakening: This choice will actually hurt the argument instead of help it to survive.
Strengthening: This choice will help the conclusion follow from its evidence, but it doesn't have to be true in order for the argument to work. Be careful—a strengthener isn't the same thing as a necessary assumption, even though the argument is somewhat "improved" in both cases!

Your turn

Necessary Assumption 1

Syndicated political columnists often use their newspaper columns to try to persuade readers to vote a certain way. However, their efforts to persuade voters rarely succeed, for by the time such a column appears, nearly all who will vote in the election will have already made a decision about which candidate to vote for.

Which one of the following is an assumption required by the argument?

(Choice A)
Syndicated columnists influence the votes of most of their readers who have not yet decided which candidate to vote for.
(Choice B)
The attempts of syndicated political columnists to persuade readers to vote a certain way in an election can instead cause them to vote a different way.
(Choice C)
People who regularly read columns by syndicated political columnists mainly read those written by columnists with whom they already largely agree.
(Choice D)
Regular readers of columns by syndicated political columnists are less likely to be persuaded to vote a certain way by such columns than are people who seldom read such columns.
(Choice E)
People rarely can be persuaded to change their minds about which candidate to vote for once they have made a decision.

There's a subtle shift in this argument: from [voters] will have already made a decision to [can't] persuade voters to vote a certain way.

If you didn't detect that gap, or are wondering how you might have found that on your own, try attacking the argument:

People could change their minds after reading an article, even if they had previously decided how they were going to vote.

Since this possibility weakens the argument, then the corresponding necessary assumption would be:

People won't change their minds on how they'll vote after reading the news article.

Is there a choice that matches this prediction?

Necessary Assumption 2

Theater managers will not rent a film if they do not believe it will generate enough total revenue—including food-and-beverage concession revenue—to yield a profit. Therefore, since film producers want their films to be shown as widely as possible, they tend to make films that theater managers consider attractive to younger audiences.

Which one of the following is an assumption required by the argument?

(Choice A)
Adults consume less of the sort of foods and beverages sold at movie concession stands than do either children or adolescents.
(Choice B)
Movies of the kinds that appeal to younger audiences almost never also appeal to older audiences.
(Choice C)
Food-and-beverage concession stands in movie theaters are usually more profitable than the movies that are shown.
(Choice D)
Theater managers generally believe that a film that is attractive to younger audiences is more likely to be profitable than other films.
(Choice E)
Films that have an appeal to older audiences almost never generate a profit for theaters that show them.

Conclusion: [Film producers] tend to make films that theater managers consider attractive to younger audiences

because

Support:

Theater managers will not rent a film if they do not believe it will generate enough total revenue—including food-and-beverage concession revenue—to yield a profit, [and]
Film producers want their films to be shown as widely as possible.

Can you detect a gap or an overlooked possibility in this argument?

You may have seen that the conclusion introduces the idea of films that are attractive to younger audiences for the first time. That means that the arguer must be drawing a connection between something in the evidence and the idea of films that are attractive to younger audiences—otherwise, it wouldn't make any sense to address this idea in the conclusion!

So, take a look at the evidence. What idea does the arguer assume to have a relationship with films that are attractive to younger audiences?

We're told that theater managers won't rent a film if they think it won't yield a profit. We're also told that film producers want their films to be shown as widely as possible, which would require theater managers to rent a film. Then, the arguer jumps to the conclusion that this is why film makers make films that are attractive to younger audiences.

One thing that the arguer must be assuming is that films that theater managers consider attractive to younger audiences are the films that will be profitable. If this weren't true, then the argument wouldn't really have a leg to stand on.

Is there a choice that expresses this concept?

Necessary Assumption 3

One-year-olds ordinarily prefer the taste of sweet food to that of salty food. Yet if one feeds a one-year-old salty food rather than sweet food, then over a period of about a year he or she will develop a taste for the salty flavor and choose to eat salty food rather than sweet food. Thus, a young child's taste preferences can be affected by the type of food he or she has been exposed to.

Which one of the following is an assumption required by the argument?

(Choice A)
Two-year-olds do not naturally prefer salty food to sweet food.
(Choice B)
A child's taste preferences usually change between age one and age two.
(Choice C)
Two-year-olds do not naturally dislike salty food so much that they would not choose it over some other foods.
(Choice D)
The salty food fed to infants in order to change their taste preferences must taste pleasant.
(Choice E)
Sweet food is better for infant development than is salty food.

Conclusion: A young child's taste preferences can be affected by the type of food he or she has been exposed to

because

Support:

One-year-olds ordinarily prefer the taste of sweet food to that of salty food, [but]
If one feeds a one-year-old salty food rather than sweet food, then over a period of about a year he or she will develop a taste for the salty flavor and choose to eat salty food rather than sweet food.

Can you identify a gap or overlooked possibility in this argument when you examine the path from support to conclusion?

To point out an overlooked possibility, we might ask:

What if the one-year-old's tastes simply changed over the period of a year that you cited?

In other words, perhaps it's not that the taste preferences were affected by specific food exposure, but that something else happened (such as mere aging) to cause the change in preference.

If the overlooked possibility is that the one-year-old's tastes could have changed over the period of a year, then how might we formulate the necessary assumption?

Some final thoughts on Necessary Assumptions

Prefer weak choices. If you are running short on time, and you've eliminated down to a choice featuring strong language (all, none, never) and a choice featuring weak language (some, sometimes, occasionally), lean towards the choice with the weaker language—necessary assumptions are more often characterized by weaker statements than sufficient assumptions are.
Practice analyzing arguments for their conclusion and evidence. This could very well be a skill that you practice all the way up until Test Day, since it's so fundamental to many question types beyond Necessary Assumptions.
Be patient. Because necessary assumptions are by definition implicit (never mentioned in the argument), it can take some time to get adept at finding them. Feel free to return to this article for a refresher if you find yourself struggling in the future.
Look for the potential "gap" in the argument. If you see an idea that appears in the conclusion but was never mentioned in the evidence, that's a great place to start—it's likely that that idea is part of the assumption.

Want to join the conversation?

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Ari S
Posted 6 years ago. Direct link to Ari S's post “At the end in the "Some f...”
At the end in the "Some final thoughts on Necessary Assumptions"
Lean towards the choice with the weaker language—necessary assumptions are more often characterized by weaker statements than sufficient assumptions are.

Can anyone explain why, what the logic would be for this?
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(8 votes)
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- Jorge Lainez
  Posted 6 years ago. Direct link to Jorge Lainez's post “Imagine an argument like ...”
  Imagine an argument like the one below.
  
  Katy: “I know it’s going to rain tomorrow in the town between these two times: 1 o’clock and 2 o’clock. I know it’s going to rain the day after tomorrow in the town between these two times: 3 o’clock and 4 o’clock. I cannot go with you, Mary, to the plaza or the park this week. I don’t have an umbrella, and I don’t have a coat; I don’t have anything that could protect me from these weather events. I know you planned these places in advanced for me, and I am sorry.”
  
  There are three necessary assumptions:
  1) Katy does not have any free time besides the time when it's going to rain on those particular days.
  2) Katy cannot, for some reason, share an umbrella with Mary.
  3) It's not possible for Mary to rearrange the times to meet with Katy.
  
  Looking at #1, you can make the necessary assumption too strong by assuming that Katy does not have any free time. However, we are only trying to bridge the gap between her main conclusion and her supporting evidence in order for it to work. If the answer were too strong, then you would be guaranteed the argument works rather than the argument must need the assumption to work.
  
  If Katy cannot go to those places in that week because of the weather events, and Katy argues she cannot protect herself from those weather events, then it is necessary to assume Katy does not have any free time besides the time when it's going to rain on those particular days, which is why Mary chose those particular times in the first place.
  
  Remember: necessary assumptions must be true for an argument to work. Sufficient assumptions guarantees the argument works.
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  (13 votes)
JR Nowlan
Posted 6 years ago. Direct link to JR Nowlan's post “Why does the passage use ...”
Why does the passage use "one-yr-olds" but switches to "two-yr-olds" in the choices in the last exercise?
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(1 vote)
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- Ariana Healy
  Posted 6 years ago. Direct link to Ariana Healy's post “It says in the passage "O...”
  It says in the passage "One-year-olds ordinarily prefer the taste of sweet food to that of salty food. Yet if one feeds a one-year-old salty food rather than sweet food, then OVER A PERIOD OF ABOUT A YEAR he or she will develop a taste for the salty flavor and choose to eat salty food rather than sweet food." so the choices are referring to when the 1-year-old becomes a 2-year-old
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  (8 votes)
Lyu Jie
Posted 4 years ago. Direct link to Lyu Jie's post “In flaw questions, we hav...”
In flaw questions, we have to assume the evidence is right and find the gap between evidence and conclusion, but can we question the validity of the evidence in assumption questions? Here is an example that the evidence is inconsistent with the context that the stimulus provides:

Museum visitors: the national government has mandated a 5% increase in the minimum wage paid to all workers. this mandate will adversely affect the museum-going public. the museum's revenue does not currently exceed its expenses, and since the mandate will significantly increase the museum's operating expense, the museum will be forced either to raise fees or to decrease services.

the question asks for a necessary assumption, my question is: the stimulus has already stated explicitly that"the mandate will significantly increase the museum's operating expense", but the right answer choice says"some of the employees are not paid more than the minimum wage", which indicates the operational expense won't be increased by the mandate, please can anyone help? thanks
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(3 votes)
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jopang
Posted 3 years ago. Direct link to jopang's post “Could anyone help? Why th...”
Could anyone help? Why the choice C of question 1 is weakening the argument? If people only read those articles from who they are largely liked, then there is no chance for them to choose another one. May be its not a necessary assumption but it's strengthen the argument.
Button navigates to signup pageComment on jopang's post “Could anyone help? Why th...”
(1 vote)
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A A
Posted 5 years ago. Direct link to A A's post “Our core is: *reducing ...”
Our core is:

reducing class size --> hire more teachers

but already too few qualified teachers

reducing class size --> more individual instruction, but education suffers

-->

reducing class size won't improve overall student achievement

We're looking for a necessary assumption--that means it has to be true. How do we test? Negate!

(E) tells us that other, qualified teachers aren't going to move into the area to fill the need for new teachers. If this were true (negating it), the argument would fall apart: reducing class size (by hiring new, qualified teachers) could lead to an overall improvement in achievement.

(A) gives us a "should." Our conclusion isn't about what should or should not happen. This "should" immediately be a red flag (har har)! To draw a conclusion about what will happen, we don't need to assume that something "should" happen.

(B) gives us the opposite of what we want. It conflicts with the conclusion.

(C) is irrelevant. We're not worried about what students value.

(D) is tempting, but it's too extreme. It wouldn't improve ANY student's achievement? That's not necessarily true. Our conclusion is about it not improving education overall--that means, in general. It still leaves room for a new plan to improve some students' performance... just not that many.
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