When 100 people who have not used cocaine are tested for : GMAT Critical Reasoning (CR)
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# When 100 people who have not used cocaine are tested for

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When 100 people who have not used cocaine are tested for [#permalink]

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14 Nov 2007, 11:23
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94. When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument
(A) attempts to infer a value judgement from purely factual premises
(B) attributes to every member of the population the properties of the average member of the population
(C) fails to take into account what proportion of the population have used cocaine
(D) ignores the fact that some cocaine users do not test positive
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine

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14 Nov 2007, 12:05
i say C
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14 Nov 2007, 12:15
eileen1017 wrote:
94. When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument
(A) attempts to infer a value judgement from purely factual premises
(B) attributes to every member of the population the properties of the average member of the population
(C) fails to take into account what proportion of the population have used cocaine
(D) ignores the fact that some cocaine users do not test positive
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine

Id say C.

B: Honestly, I can't really make sense of this one. I think it means that all people are equal in the population. Im not really sure so I didnt essentially rule this out, but chose C over this.

A: No way.
D: Doesnt ignore this fact at all. In fact it uses this premise for its argument.
E: Does not advocate. This is never suggested.
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14 Nov 2007, 16:46
[quote="eileen1017"]
(A) attempts to infer a value judgement from purely factual premises
out of scope

(B) attributes to every member of the population the properties of the average member of the population
Doesn't disqualify conclusion.

(C) fails to take into account what proportion of the population have used cocaine
Correct. Conclusion says that the majority of people who tested positive will be people who used cocaine. This is true when, for example, 50% of the population use cocaine. But can't be true if only 0.00005% use cocaine.

(D) ignores the fact that some cocaine users do not test positive
This is taken into consideration. Number = 1%

(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine
Out of scope
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15 Nov 2007, 07:56
I would say C.
Lets say 200 people are randomly chosen for the survey and out of this 199 are non-cocaine users and one person uses cocaine.
SO lets us assume that approximately 11 people test positive -- 10(non users) + 1 (user).
Here the majority of those tester +ve are non-users. This will completely shift if we change the ratio and make it 199 users and 1 non-user.
So it is important to know the percentage of cocaine users in that population.
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15 Nov 2007, 07:58
Thanks to all. The OA is C.
15 Nov 2007, 07:58
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