lsat-3-1-23

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

看得直晕,正确答案是(c),why?请大家指教.

题干的结论可能错的原因可以一个极端的例子看出:

RANDOMLY CHOSEN 1000个人去TEST, 这1000个人刚好都不是COCAINE USERS(不是没有可能), 结果很可能是50个人POSITIVE, 所以不能说the vast majority of those who test positive will be people who have used cocaine

原来如此,

我懂了,

谢谢版主!