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November 20, 2009

Difference Between Necessary & Sufficient Assumption Questions

LSAT Blog Difference Necessary Sufficient Assumption QuestionsBoth Necessary Assumption and Sufficient Assumption questions contain the word "assumption" within the question stem. However, they ask for very different things.

(Also see Sufficient Assumption Questions and the Negation Test.)

This is why it's misleading (and often confusing) to simply refer to both types under the catch-all category of "Assumption" questions (as Kaplan does, for example).

Another company refers to Necessary Assumption questions as "Assumption" questions, and it refers to Sufficient Assumption questions as "Justify" questions. However, referring to Sufficient Assumption questions as "Justify" questions is often confusing for test-takers for two major reasons.

Many test-takers mistakenly believe:

1. The question stem must contain the word "justify" in order to be a "Justify" question (i.e. they believe the word "justify" is a necessary condition for it to be a sufficient assumption question).

In fact, Sufficient Assumption questions rarely contain the word "justify."


2. Any question stem containing the word "justify" is a "Justify" question (i.e. they believe the word "justify" is a sufficient condition for it to be a sufficient assumption question).

In fact, "Strengthen" and "Strengthen Principle" questions often contain the word "justify" as well.


Basically, the presence of the word "assumption" doesn't tell you what type of question you're dealing with, and neither does the word "justify".

So...how do you tell the difference?

Instead of focusing on the noun "assumption," focus on the verb in the question stem.

The following indicators are presented in a rough decreasing order of frequency as they appear in PrepTests 50-58, whenever possible. If your materials refer to PrepTests by month and year instead, please see this list.


Necessary Assumption questions often contain the words:

"Depends"
Example #1: "Which one of the following is an assumption that the fund-raiser's argument depends on?"
(PrepTest 56, Section 3, Question 18)

Example #2: "Which one of the following is an assumption on which the argument depends for its conclusion to be properly drawn?"
(PrepTest 52, Section 1, Question 10)


"Requires"
Example: "Which one of the following is an assumption the bureaucrat's argument requires?"
(PrepTest 55, Section 3, Question 19)


"Required"
Example: "Which one of the following is an assumption required by the dietitian's argument?"
(PrepTest 56, Section 2, Question 6)


"Relies"
Example: "Which one of the following is an assumption on which the argument relies?" (PrepTest 56, Section 3, Question 9)


The following three are significantly less common:

"Makes"
Example: "The argument makes which one of the following assumptions?"
(PrepTest 56, Section 2, Question 4)


"Necessary"
Example: "Which one of the following is an assumption necessary for the critic's conclusion to be properly drawn?"
(PrepTest 55, Section 1, Question 12)

(Granted, the word "necessary" is not a verb, but it's a pretty telling indicator that this is a necessary assumption question. If you really want a verb there, we could substitute the word "required" for "necessary.")


"Assumes"
Example: "The argument assumes that"
(PrepTest 30, Section 2, Question 15 - page 58 in Next 10)



Sufficient Assumption questions often contain the words:

"Follows logically if...assumed"
Example #1: "The argument's conclusion follows logically if which one of the following is assumed?"
(June 2007 LSAT, Section 2, Question 6)

Example #2: "The critic's conclusion logically follows if which one of the following is assumed?" (PrepTest 56, Section 2, Question 10)

(The above language is an extremely common indicator of sufficient assumption questions, so I'm including two examples. Yes, LSAC can play with the order of words. They do so in the following example as well.)


"Properly inferred if...assumed"
Example: "Which one of the following, if assumed, allows the conclusion of the therapist's argument to be properly inferred?"
(PrepTest 56, Section 3, Question 16)


"Properly drawn if...assumed"
Example: "The argument's conclusion is properly drawn if which one of the following is assumed?"
(PrepTest 55, Section 1, Question 4)


"Enables"
Example: "Which one of the following, if assumed, enables the conclusion of the argument to be properly drawn?"
(PrepTest 54, Section 4, Question 22)


"Allows"
Example: "Which one of the following is an assumption that would allow the conclusion above to be properly drawn?"
(June 2007 LSAT, Section 2, Question 15)


"Justifies"
Example: "Which one of the following is an assumption that would serve to justify the conclusion above?"
(PrepTest 4, Section 1, Question 2 in LSAT TriplePrep, Volume 1, one of the books of rare PrepTests.)


***

By the way, here's how to tell a Strengthen Principle question from a Sufficient Assumption (Justify) question when both contain some version of the word "justify":

A Strengthen question says something like, "most helps to justify." The "most helps" part indicates you're looking to strengthen it --- to some degree.

Example #1: "Which one of the following principles most helps to justify the mathematics teacher's argument?"
(PrepTest 55, Section 1, Question 6)

Example #2: "Which one of the following, if assumed, most helps to justify the reasoning above?"
(PrepTest 50, Section 4, Question 9)

A Sufficient Assumption (Justify) question asks you to "justify the conclusion" - meaning the correct answer will justify it all the way --- 100%.


***

Also see: Arguments and Contrapositives | Necessary and Sufficient Assumptions

Photo by kharied



10 comments:

  1. Thank you so much for posting! I noticed these discrepancies while studying with my Kaplan and Power score books. It is great to know that I am not going crazy! Thanks for the clarification! :)

    ReplyDelete
  2. This is a fantastic post, because it does what PowerScore does not: it puts the two question types in a directly comparative light. I've been scoring well on LR practice tests (-3 avg per section) until now, but the vast majority of my missed answers have been on Assumption and Justify questions. This post has allowed me to recognize the distinction between the two question types--they are quite different in nature.

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  3. Hi Steve,

    I realize you wrote this half a year ago, but I had a quick question and also wanted to thank you for all your hard work on this blog. Posts like this one are invaluable.

    My question is -- when you said this was necessary:

    "Makes"
    Example: "The argument makes which one of the following assumptions?"
    (PrepTest 56, Section 2, Question 4)

    It could also be sufficient, right? Not necessarily sufficient but possibly.

    ReplyDelete
  4. Glad you're enjoying the blog!

    The question stem you refer to is asking for a necessary assumption - one that is contained within the argument.

    If you say to someone, "you're making an assumption", you're referring to an assumption they've definitely made and would have to be true in order for their argument to be valid (necessary).

    You wouldn't be referring to something that, if true, would fully justify their argument (sufficient).

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  5. I rly appreciate your help! Why is the LSAT so hard? :(

    ReplyDelete
  6. Hi Steve - one of the key identifiers listed in the LRB is that any necessary assumption question (they call it "assumption") never uses the word "if" (or any other sufficient condition indicator) in the question stem. I still find that to be the easiest way to determine whether I am dealing with a necessary or sufficient assumption question. When there's no "if" (or similar), I know it's a necessary assumption, even if it uses the word "justify".

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  7. Steve - wow ...please, why has Kaplan not been sued?! Their explanations are horrible, and their conflation of sufficient and necessary here invalidates all of their materials in my mind. HOW could they do this?! An answer choice can be sufficient without being necessary; its negation could even be OK! And yet Kaplan even applies the denial test to find correct answers in sufficient assumption questions.

    There are multiple ways to make an argument follow!

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  8. PT 34, Section 2, #5. Answer choice A is more than sufficient to make the argument valid, but not necessary. Answer choice D is necessary. If you don't understand diff between nec. and suf. assumption questions, you're in trouble!

    ReplyDelete
  9. i am always getting at least two NA questions wrong on all the test! it is my whole. i have tried these until i am blue in the face. i am always getting between 17-18 on the LR, and this is mostly due to NA,inference, and conditionals! ahwww. :-/ i have driven myself crazy!

    ReplyDelete
  10. Hi Steve,

    For preptest 5, LR1, question 12. I treated the question stem as sufficient, and got the question wrong. It seems to me, that this question stem indicates that this is a sufficient question ("properly inferred...if"), however, the answer is a necessary condition that must be satisfied. Am I incorrect?

    Thanks

    ReplyDelete