求助--费费逻辑94

apanapan · 发表于 2004-3-17 04:33:00

94. 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 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 judgement 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

正确答案是C)可是怎么也和题目挂不上钩。私下盼望D)如果是ignores the fact that some people test positive even they never used cocaine 那我就毫不犹豫地选它。可惜不是。这道题该怎么考虑请指教。

maotian · 发表于 2004-3-17 10:16:00

以下是引用apanapan在2004-3-17 4:33:00的发言：

94. 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 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 judgement 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

正确答案是C)可是怎么也和题目挂不上钩。私下盼望D)如果是ignores the fact that some people test positive even they never used cocaine 那我就毫不犹豫地选它。可惜不是。这道题该怎么考虑请指教。

这题我刚做过，赫赫。

虽然没用过cocaine的人5%positive,用过的人99%positive.

但是注意，对整个随机人群（含有两类人：用过的和没用过的）进行测试时，应注意这两类人的比例。eg:如果用过的占整人群1%，而没用过的占99%，即使前者positive的人比例为0，后者positive的比例为100%,总体中positive的人也不过是1%。

D选项是个作用不确定选项，因为有一些测试错误，不一定会导致结论不成立。上面这个例子也可以证明这一点。

apanapan · 发表于 2004-3-17 11:56:00

谢谢你,maotian

看了你的解释,觉得好像明白了,可是脑袋里的结还是没能全解开。后天考试了,希望运气好,不要到时大脑停止运作。

rillshiegin · 发表于 2011-7-13 19:18:33

"Thus, when a randomly chosen group of people is tested for cocaine use, the vast people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine. "
这句话是结论，结论认为：一个随机测试，如果绝大多数人的cocaine最后检测结果是阳性，则这些人一定是使用了cocaine的那部分人。但是argument里少考虑了一个问题：打个比方，100个人里，如果没有服用cocaine的人，占样本人群的99%，那么最终这些没有服用cocaine但是还是结果呈阳性的人，为4个人；而剩下的1%，即1个人，这1个人的99%是服用了cocaine并且结果呈现阳性的人，即0个人。这样子的话，绝大多数实验结果为阳性的人群则是那些没有服用cocaine的人咯~~~

所以C是对的：作者没有考虑到，那些真正服用了cocaine的人们，占总人群的人数

an551982 · 发表于 2016-9-23 16:34:48

刚好也做错了这道题。网上找了答案大概明白了大牛的思路。
我觉得这道题最要紧的是读懂结论这句话：
Thus, when a randomly chosen group of people is tested for cocaine use, the vast people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
尤其是最后半句，意思是大部分测试结果为阳性的测试者是真正在用可卡因的人。
所以整个题目的逻辑是：
前提：
没用可卡因的人里测试出阳性比列是100人中有5人，用了可卡因的人里阳性比例是100人中有99人。
结论：
一群人被随机抽选去接受可卡因测试，测出阳性的大部分都是真正在用可卡因的人。

这样就知道这个逻辑的问题在哪里了，就是用可卡因和没用可卡因的人的比例问题。
假设这群人里没用可卡因的人数大大超出用可卡因的人数，则仍然有可能被查出阳性的人数（虽然比例很低）里没用可卡因的比在用可卡因的（虽然比例很高）多。这样题目结论就被推翻了。
所以答案是C。

求助--费费逻辑94

求助--费费逻辑94

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