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Cocaine

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by sogmat » Wed Apr 15, 2009 6:48 pm
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

OA -C
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Source: — Critical Reasoning |

by delhiboy1979 » Thu Apr 16, 2009 12:27 am
Good question. Can someone explain what B and C mean. I am sure one them is correct.
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by cm47323 » Thu Apr 16, 2009 12:59 am
The correct answer is C. This is a classic Bayesian probability issue. If you recognize this problem as such, you'll prephrase the correct answer and be done. If not, try process of elimination:

A. no value judgment
B. tougher to eliminate. in this case, this error in reasoning does not apply since it is a binary situation (user, nonuser). An example of this flaw would be "The poverty line is defined as household income of less than $25,000. The average household income is $45,000 in the U.S., so nobody lives below the poverty line."
C. correct answer
D. clearly false, it acknowledges 1/100 users do not test positive
E. much like there is no value judgment, there is no advocacy

Source? Seems like an LSAT question to me.
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by samanthaJ79 » Sun May 15, 2016 4:47 am
I also think that the right answer is C
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