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

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Director
Joined: 01 Feb 2003
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When 100 people who have not used cocaine are tested for [#permalink]  23 Apr 2005, 15:56
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Difficulty:

5% (low)

Question Stats:

0% (00:00) correct 100% (00:00) wrong based on 1 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.
Director
Joined: 03 Nov 2004
Posts: 870
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I will go with 'A'

The assumption is that the randomly chosen group will be of people who have used cocaine.
Director
Joined: 04 Jul 2004
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(C) for me.

Author is considering equal number of ppl have taken cocain, which is wrong.
VP
Joined: 26 Apr 2004
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Location: Taiwan
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Re: CR: Cocaine [#permalink]  24 Apr 2005, 05:09
Vithal 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 for me.

(1) When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive.

(2) 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.

we don't know whether the randomly chosen group has used cocaine or not.

If the randomly chosen group of people are picked from the 5 people who have not used cocaine but have positive result.
We cannot reach the conclusion.
Director
Joined: 01 Feb 2003
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OA is C indeed!
Manager
Joined: 03 Nov 2004
Posts: 193
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chunjuwa though your explanation is correct , but is it what C is trying to say.

may be that i am misunderstanding C.

any one would please explain what C is implying
Senior Manager
Joined: 07 Nov 2004
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Can someone tell me why D is wrong?
Senior Manager
Joined: 12 Oct 2003
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Location: sydney
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Thats was a toughie.... i chose A earlier. now i understood why C is right.

lets take numbers

Cocain people +

user 100 99

Non user 100 5

Random sample has 10% users and 90% non-users then more people will be test +ve, if randon sample contains 90 non-users and 10% used less people wil be tested +ve. therefore conclusion does not consider proportion.
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