feifei-94

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 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

看了狒狒逻辑详解，可是还是不太懂此题……

nonuser里面，5%的人会被test positive

user里面，99%的人会被test positve

蓝色部分读不太明白，是不是：一大帮来test，会有人被查出来positive。这群positive的人里面自然有真有假。但是绝大部分会是真的user。

不太明白。请NN详解。谢谢……

CR will get more interesting when math is involved. The most seen math problem in GMAT CR is percentage. What ETS wants to test is whether you can distinguish between absolute value and relative value, e.g. percentage.

What you need to remember is that higher percentage in CR does not indicate that the absolution number or quantity will be higher compared to lower percentage. To the contrary, higher percentage always has smaller numbers or quantity.

In this question, the conclusion is the last sentence. The premises are that 5% of non-users and 99% of users are tested positive. The argument fails to consider the composition of population and makes an unwarranted assumption that among a random group of people, non-users are less than 99/5 of users. X: non-users Y: users. 5%X < 99%Y è x<99/5Y. If this proportion is not true, the argument does not hold.

So you need to know whether the question is about absolute value or relative value. If it is about relative value, you need to know what the BASE is. If comparison is involved, you need to find out the relationship between the bases.

你根本不知道随机调查的那帮人的组成。然而，这个结论却用了个强调必然性的语句（这点能够从"the vast majority..will.."中看出）。

那请问这道题应该选什么呢？

不解中。。