Help?LSAT Section3-I-23

bigmouse · 发表于 2003-9-4 11:03:00

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.
answer:C my choice:B
why?

braveMBA · 发表于 2003-9-4 11:38:00

B是倒装，还原后是attributes the properties of the average member of the population to every member of the population，似乎这并不是推理的ERROR，因为结论说the vast majority of those who test positive，同整体的every member of the population没有什么关系，是特定人群。
C指出了一个推理谬误，因为如果整体人群中have used cocaine的人数很少的话，例如，就100个，而整体人数是100000000000，则被randomly chosen group的概率变得很低，也就无法推出the vast majority of those who test positive will be people who have used cocaine.

bigmouse · 发表于 2003-9-4 16:08:00

thanks!明白了！

Help?LSAT Section3-I-23

Help?LSAT Section3-I-23

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