## How to solve necessary assumption?

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alexjinye

Posts: 142
Joined: Mon Jun 17, 2013 2:44 am

### How to solve necessary assumption?

I seem to have some problems solving necessary assumption questions.
Usually, when I encounter assumption questions (necessary/sufficient), I have two ways to attack them, which are drawing out a formal logic diagram or using the negation test.
Whereas formal logic works perfectly for sufficient assumption question, I always have a hard time deciding the strategy for necessary assumption questions. Sometimes formal logic works better, the other times negation test works better for necessary assumption questions.
So, how do you guys process necessary assumption questions? And how should I refine my strategy for necessary assumption questions?

The Mixed Tape

Posts: 5737
Joined: Thu Nov 06, 2014 4:14 pm

### Re: How to solve necessary assumption?

always negation test

alexjinye

Posts: 142
Joined: Mon Jun 17, 2013 2:44 am

### Re: How to solve necessary assumption?

ok. thx for the input.
I will drill some more necessary assumption questions tonight by using the negation test only. Maybe draw out formal logic for necessary assumption is not a good idea?

alexjinye

Posts: 142
Joined: Mon Jun 17, 2013 2:44 am

### Re: How to solve necessary assumption?

The Mixed Tape wrote:always negation test

gotya!
drilling time!

Ron (PowerScore)

Posts: 21
Joined: Wed Oct 01, 2014 6:13 pm

### Re: How to solve necessary assumption?

The best approach is to focus on the conclusion and generate a solid prephrase for both the correct and the incorrect answer choices. For example, if the conclusion has out-of-nowhere information that wasn't in a premise, the correct answer choice will tend to link that new information to the premises. If the conclusion doesn't have out-of-nowhere information in it, then look for the logical flaw in the argument and prephrase that the correct answer choice will defend the conclusion against an attack on that area of weakness.

For the incorrect answer choices, the best way to think about them is much like you would in a Must Be True question. Since it's a necessary assumption, the correct answer choice must be part of the argument if the conclusion is valid. Be suspicious of strongly worded or highly restrictive answer choices, or answer choices that bring in new information only tangentially related to the conclusion.

I know it's a bit difficult to grasp in the abstract, but if you want to reference a specific question you're having trouble with, I'd be happy to walk you through the process.

Ron

kray

Posts: 130
Joined: Sat Feb 21, 2015 8:54 pm

### Re: How to solve necessary assumption?

alexjinye wrote:I seem to have some problems solving necessary assumption questions.
Usually, when I encounter assumption questions (necessary/sufficient), I have two ways to attack them, which are drawing out a formal logic diagram or using the negation test.
Whereas formal logic works perfectly for sufficient assumption question, I always have a hard time deciding the strategy for necessary assumption questions. Sometimes formal logic works better, the other times negation test works better for necessary assumption questions.
So, how do you guys process necessary assumption questions? And how should I refine my strategy for necessary assumption questions?

Just my approach:

As I read the question I ask myself, "What HAS to be true in order for the conclusion to make sense?" -- maybe one or two things will come to mind, and I'll keep those in the back of my head as I go through and do negation test. Each possible answer, I look at and imagine ANY POSSIBLE SCENARIO where the conclusion could still follow even if that answer choice didn't exist. If there is ANY CHANCE, cross if off and next choice. Often, very "strong" answer choices are wrong for necessary assumption questions... Answers with words like "always" or "all". Having possibilities in mind helps identify potential correct answers more quickly, but still do due diligence of making sure they're actually saying what you think they are.

KDLMaj

Posts: 145
Joined: Mon Jun 30, 2008 4:07 pm

### Re: How to solve necessary assumption?

kray wrote:
alexjinye wrote:I seem to have some problems solving necessary assumption questions.
Usually, when I encounter assumption questions (necessary/sufficient), I have two ways to attack them, which are drawing out a formal logic diagram or using the negation test.
Whereas formal logic works perfectly for sufficient assumption question, I always have a hard time deciding the strategy for necessary assumption questions. Sometimes formal logic works better, the other times negation test works better for necessary assumption questions.
So, how do you guys process necessary assumption questions? And how should I refine my strategy for necessary assumption questions?

Just my approach:

As I read the question I ask myself, "What HAS to be true in order for the conclusion to make sense?" -- maybe one or two things will come to mind, and I'll keep those in the back of my head as I go through and do negation test. Each possible answer, I look at and imagine ANY POSSIBLE SCENARIO where the conclusion could still follow even if that answer choice didn't exist. If there is ANY CHANCE, cross if off and next choice. Often, very "strong" answer choices are wrong for necessary assumption questions... Answers with words like "always" or "all". Having possibilities in mind helps identify potential correct answers more quickly, but still do due diligence of making sure they're actually saying what you think they are.

Super late on this one, but you really have to figure out what kind of argument you're in. Your two big buckets are Overlooked Possibilities and Scope Shifts.

As one poster put it, if the problem is that the author has introduced something new in the conclusion that isn't related to their evidence, then you're going to look for an answer choice that has both the mismatched concept from the conclusion and the one in the evidence that should have been linked to it to begin with. It also must have the right relationship (usually the two are alike in some way, they are mutually exclusive or dissimilar in some way, or you need the evidence concept for the conclusion concept). In this instance, any answer choice that brings up *anything* from outside of the stimulus is 100% incorrect. It makes for a fast elimination.

i.e.

I'm not a great LSAT instructor because I am tall

A. LSAT Instructors are taller than GRE Instructors
B. Being tall makes one unsuitable for many teaching positions
C. Being tall interferes with one's ability to excel at teaching the LSAT at least some of the time
D. Being tall interferes with one's ability to excel at teaching the LSAT all of the time
E. Tall people are legally allowed to teach prep tests.

Being tall has nothing to do with being an LSAT instructor. Also the format of the argument: ~X because Y tells me that the author is assuming being an LSAT instructor is mutually exclusive with being tall. Correct answer MUST include: being tall and being an LSAT instructor, and it CANNOT mention any concepts not mentioned in the stimulus. So:

A. LSAT Instructors are taller than GRE Instructors [Eliminate]
B. Being tall makes one unsuitable for many teaching positions [Eliminate]
C. Being tall interferes with one's ability to excel at teaching the LSAT at least some of the time
D. Being tall interferes with one's ability to excel at teaching the LSAT all of the time
E. Tall people are legally allowed to teach prep tests. [Eliminate]

And the answer MUST have the right relationship (and degree) for the argument.

A. LSAT Instructors are taller than GRE Instructors [Eliminate]
B. Being tall makes one unsuitable for many teaching positions [Eliminate]
C. Being tall interferes with one's ability to excel at teaching the LSAT at least some of the time [Correct]
D. Being tall interferes with one's ability to excel at teaching the LSAT all of the time [Eliminate]
E. Tall people are legally allowed to teach prep tests. [Eliminate]

The argument doesn't require that being tall interferes ALL of the time. Just enough that you would no longer qualify as great. If it were a sufficient assumption question, the "all of the time" wouldn't be a problem.

If it's an overlooked possibilities issue- the evidence is the right kind of evidence, but the conclusion is going too far. In that case, the correct answer is going to rule out one (or more) unconsidered possibilities that- if true- would crash the conclusion. So if an answer choice has something from outside of the stimulus, do NOT automatically discard it. The answer choices in these are almost always directly in the negative (X DIDN'T happen, Y ISN'T the case etc)- so the denial test is relatively easy to use.

I can't find my car. Obviously, it was stolen.

Not being able to find your car is absolutely relevant to the determination that it was stolen. But it doesn't prove that the car was stolen. You know this isn't a scope shift because if we softened the conclusion a bit:

I can't find my car. Obviously, it could have been stolen.

There would be no problem with the argument. The author is assuming that there's no other explanation for the fact that they can't find the car. There are an infinite number of potential explanations- I forgot where it was, it was moved by a neighbor to avoid being impounded on a street cleaning day, alien invaders swooped down and made the car invisible, etc. If even ONE of these is true, then the conclusion goes to hell. So:

A. The author can, in fact, find the car.
B. Alien space invaders did not swoop down and make the car invisible, making it impossible for the author to find it.
C. Red cars are stolen more often than every other color of car.
D. No cars have been stolen in the area this past month.
E. Stolen cars are often found within a week of their disappearance.

If we killed anything with new information in it, we'd miss the right answer. Instead, we have to go through the answer choices and look for something that rules out a competing explanation for why they can't find the car (aside from theft).

A. The author can, in fact, find the car. [Eliminate- contradicts the evidence]
B. Alien space invaders did not swoop down and make the car invisible, making it impossible for the author to find it.
C. Red cars are stolen more often than every other color of car. [Beware the word "than" in an answer choice- irrelevant comparisons are VERY common. We didn't compare anything in the stim, and so we can't do it in the answers]
D. No cars have been stolen in the area two months ago. [This does not rule out an alternative explanation for the car's disappearance. Also watch out for answers in the wrong time frame]
E. Stolen cars are often found within a week of their disappearance. [Does not rule out any explanation for the car's disappearance other than theft]

Leaves us with B- which would have been 100% incorrect in a scope shift argument but which is wholly related to this one. Using the denial test, if, in fact, Aliens DID swoop down and make the car invisible, then we can't say for sure that it was stolen. This is correct. You'll also notice that in these kinds of arguments, you tend to have more answer choices in the negative. (Please don't use that as a way to ID an argument though) But it tells you a lot about how the LSAT approaches these arguments.

The two different subtypes is likely why you're finding your reliance on FL works sometimes but not in others. (It could go either way, but I'm going to guess it's working best when it's a scope shift) Hope that helps a bit.

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