Critical Reasoning

Expert: What criteria distinguish addictive substances from nonaddictive ones? Some have suggested that any substance that at least some habitual users can cease to use is nonaddictive. However, if this is taken to be the sole criterion of non addictiveness, some substances that most medical experts classify as prime examples of addictive substances would be properly deemed nonaddictive. Any adequate set of criteria for determining a substance's addictiveness must embody the view, held by these medical experts, that a substance is addictive only if withdrawal from its habitual use causes most users extreme psychological and physiological difficulty.

Which one of the following can be properly inferred from the expert's statements?

(A) If a person experiences extreme psychological and physiological difficulty in ceasing to use a substance habitually, that substance is addictive.
(B) Fewer substances would be deemed addictive than are deemed so at present if an adequate definition of "addictive" were employed.
(C) A substance that some habitual users can cease to use with little or no psychological or physiological difficulty is addictive only if that is not true for most habitual users.
(D) A chemical substance habitually used by a person throughout life without significant psychological or physiological difficulty is nonaddictive.
(E) "Addiction" is a term that is impossible to define with precision.

OA:[spoiler] C[/spoiler]

I received a PM requesting that I explain why A is incorrect.

A only if B means IF A, THEN B.
Mary will attend the party only if John attends the party.
Conveyed meaning:
Mary cannot attend the party on her own.
She will attend the party ONLY IF JOHN ALSO ATTENDS THE PARTY.
Thus, if Mary is seen at the party, then John must also be at the party, since Mary cannot attend without John.
In other words:
If Mary attends the party, then John attends the party.

Passage:
A substance is addictive only if withdrawal from its habitual use causes most users extreme psychological and physiological difficulty.
In other words:
If a substance is addictive, then withdrawal from its habitual use causes most users extreme psychological and physiological difficulty.
Option A incorrectly reverses the if-then statement in blue:
If a person experiences extreme psychological and physiological difficulty in ceasing to use a substance habitually, [then] that substance is addictive.
Eliminate A.

GMATGuruNY wrote:I received a PM requesting that I explain why A is incorrect.

A only if B means IF A, THEN B.
Mary will attend the party only if John attends the party.
Conveyed meaning:
Mary cannot attend the party on her own.
She will attend the party ONLY IF JOHN ALSO ATTENDS THE PARTY.
Thus, if Mary is seen at the party, then John must also be at the party, since Mary cannot attend without John.
In other words:
If Mary attends the party, then John attends the party.

Passage:
A substance is addictive only if withdrawal from its habitual use causes most users extreme psychological and physiological difficulty.
In other words:
If a substance is addictive, then withdrawal from its habitual use causes most users extreme psychological and physiological difficulty.
Option A incorrectly reverses the if-then statement in blue:
If a person experiences extreme psychological and physiological difficulty in ceasing to use a substance habitually, [then] that substance is addictive.
Eliminate A.

Dear GMATGuru,

'only if' talks about necessity while 'if' talks about sufficiency.

Are they equal? should the correct choice include 'only if'? If choice A was not reversed, could t be the correct answer?

The above is from LSAT, does the same concept tested in GMAT inference questions?

Thanks

Mo2men wrote:Dear GMATGuru,

'only if' talks about necessity while 'if' talks about sufficiency.[

Are they equal?

A only if B = If A, then B.
Both statements convey the same two meanings:
A is SUFFICIENT for B.
B is NECESSARY for A.

If choice A was not reversed, could t be the correct answer?

No.
Proposed revision of A:
A person will experience extreme psychological and physiological difficulty in ceasing to use a substance habitually if that substance is addictive.
This revision implies that -- if a substance is addictive -- then ALL PEOPLE will experience difficulty in ceasing to use it habitually.
However, the passage states that -- if a substance is addictive -- then withdrawal from its habitual use causes MOST users difficulty.
Since ALL ≠ MOST, the proposed revision of A is not supported by the passage.

The above is from LSAT, does the same concept tested in GMAT inference questions?

Thanks

The concepts tested here are far more relevant to the LSAT than to the GMAT.

@ Mo2men - What is the source ? (As Mitch pointed out, I guess it's unlikely to be an Official GMAT CR!)

@ beatthegmat: Hey Sorin, can we please make it as "best practice" to post the source in order to create a new thread in the both Verbal & Quant forums ? It'll be good to have, I think!

Hi guys,
Can anyone please shed light on the SOURCE of this CR ?

RBBmba@2014 wrote:Hi guys,
Can anyone please shed light on the SOURCE of this CR ?

It's an LSAT question.