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

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Manager
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When 100 people who have not used cocaine are tested for [#permalink]  13 Jun 2011, 07:46
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Difficulty:

5% (low)

Question Stats:

36% (01:40) correct 63% (01:09) wrong based on 11 sessions
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 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 judgment 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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Re: CR question - cocaine use [#permalink]  13 Jun 2011, 09:17
pkmme wrote:
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 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 judgment 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

C is correct because if the randomly selected people were all non-cocaine users, then 5% will be falsely detected as cocaine-users. And since all these falsely detected people have never used cocaine, the conclusion, which says most will be drug-users, breaks. Thus, it is important to consider proportion of the population who have actually used cocaine.
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Re: CR question - cocaine use [#permalink]  13 Jun 2011, 10:46
C for me as well

Crick
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Re: CR question - cocaine use [#permalink]  13 Jun 2011, 12:01
Expert's post
C as well is the only that has a flaw
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Re: CR question - cocaine use [#permalink]  14 Jun 2011, 09:19
Intern
Joined: 16 Apr 2010
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Location: United States (AL)
Concentration: Technology, Entrepreneurship
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Re: CR question - cocaine use [#permalink]  14 Jun 2011, 09:24
def C
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Re: CR question - cocaine use [#permalink]  14 Jun 2011, 15:12
I'm with C ...
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Re: CR question - cocaine use [#permalink]  14 Jun 2011, 16:19
c as well
Re: CR question - cocaine use   [#permalink] 14 Jun 2011, 16:19
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