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

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

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19 Nov 2009, 09:25
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Difficulty:

(N/A)

Question Stats:

67% (02:37) correct 33% (01:52) wrong based on 6 sessions

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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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19 Nov 2009, 09:34
gurpreet07 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

Clearly B.

100 people who have not used cocaine --> doesn't represent 10 million people who haven't used..
May be these 100 people has used cocaine in the past history... ?
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Senior Manager
Joined: 23 May 2008
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19 Nov 2009, 09:48
x2suresh wrote:
gurpreet07 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

Clearly B.

100 people who have not used cocaine --> doesn't represent 10 million people who haven't used..
May be these 100 people has used cocaine in the past history... ?

Nopes its not the correct answer
you can give it a shot again...
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19 Nov 2009, 10:46
gurpreet07 wrote:
x2suresh wrote:
gurpreet07 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

Clearly B.

100 people who have not used cocaine --> doesn't represent 10 million people who haven't used..
May be these 100 people has used cocaine in the past history... ?

Nopes its not the correct answer
you can give it a shot again...

You are right!! That's stupid mistake.. Didn't read it properly. it's clearly mentioned for every 100 ... They consider on average... My logic.. totally wrong..

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.
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19 Nov 2009, 10:51

300 members - not used --> 15 (tested +ve)
10 member - used --> 9(tested +ve)
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19 Nov 2009, 10:58
x2suresh wrote:

300 members - not used --> 15 (tested +ve)
10 member - used --> 9(tested +ve)

Now you got it right
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19 Nov 2009, 19:19
This is a kind of false positive consideration that has an applied use in real world situations (e.g. testing for hiv).
In reality, the likelihood of testing positive AND actually being positive for what is tested for, is overshadowed by the likelihood of testing positive in error; in these cases, a so called "positive" is treated with caution, even assumed to be an error based on inherent probability, until it can be confirmed valid by further testing.
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19 Nov 2009, 23:25
M1santhrope wrote:
This is a kind of false positive consideration that has an applied use in real world situations (e.g. testing for hiv).
In reality, the likelihood of testing positive AND actually being positive for what is tested for, is overshadowed by the likelihood of testing positive in error; in these cases, a so called "positive" is treated with caution, even assumed to be an error based on inherent probability, until it can be confirmed valid by further testing.

wowww what was that
sorry i din't get it...
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20 Nov 2009, 05:49
can you please explain???.i don't get it
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Re: LSAT CR   [#permalink] 20 Nov 2009, 05:49
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