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

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Director
Joined: 20 Apr 2005
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When 100 people who have not used cocaine are tested for [#permalink]  09 May 2005, 17:23
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23. 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
Manager
Joined: 28 Jan 2005
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Answer is D. If some ppl who use Cocaine DONT test positive and also 5% of those who dont test positive then the argument can fall apart.
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1) 100 pp who have not used cocaine tested for coacaine use: 5 test positive
2) 100 pp who have used cocaine tested for cocaine use: 99 test positivie
3) In a randomly chosen group, vast majority of those who test positive are pp who have used cocaine. <--conclusion

A reasoning error in the argument is that the argument

(A) attempts to infer a value judgment from purely factual premises
- Not sure what a value judgment is, but rule it out as I do not think the author is making any form of judegment

(B) attributes to every member of the population the properties of the average member of the population
- I'll take this. He used the survey results (premise 1 and 2) and impose it as a general rule for a random group.

(C) fails to take into account what proportion of the population have used cocaine
- Knowing the proportion is not impt

(D) ignores the fact that some cocaine users do not test positive
- The author did not ignore this fact. If he did, he would not have used the term 'vast majority'

(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine
- Out of scope

I'll go for B.
VP
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C)...the author wants to test the use of cocaine. then he says that the majority will test positive. in the argument the majority that tested positive was from a group of people who used cocaine. so when we know the proportion of cocaine users of the randomly tested group, we can say that the majority will test positive because it is consistent with the facts of the arguement. unless we dont know the proportion of cocaine users, we cannot support the conclusion of the author.
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Re: CR: Cocaine Testing [#permalink]  10 May 2005, 12:37
Its a straightforward C.

Since 5% of non users test positive, and 99% of users test positive, as long as users are less than 5.05%, more non users would test positive than users, in a sufficiently large (ideally infinitely large sample).
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Re: CR: Cocaine Testing [#permalink]  10 May 2005, 13:21
kapslock wrote:
Its a straightforward C.

Since 5% of non users test positive, and 99% of users test positive, as long as users are less than 5.05%, more non users would test positive than users, in a sufficiently large (ideally infinitely large sample).

In continuation...

Thefore, since the proportion of users and non users are not provided, we cannot conclude.
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Director
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The OA is C.
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