费费94题

cecily0601

求大神解救！94. When 100 people who have notused 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 vastpeople is tested for cocaine use, the vast majority of those who test positivewill be people who have used cocaine.


A reasoning error in the argumentis that the argument

A.       attemptsto infer a value judgment from purely factual premises
B.       attributes to every member of the population theproperties of the average member of the population
C.       fails to take into account what proportion ofthe population have used cocaine
D.       ignoresthe fact that some cocaine users do not test positive
E.       advocates testing people for cocaine use whenthere is no reason to suspect that they have used cocaine

CR will get moreinteresting 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 valueand relative value, e.g. percentage.
What you need toremember is that higher percentage in CR does not indicate that the absolutionnumber or quantity will be higher compared to lower percentage. To thecontrary, 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 and99% of users are tested positive. The argument fails to consider thecomposition of population and makes an unwarranted assumption that among arandom 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 thisproportion is not true, the argument does not hold.
So you need toknow whether the question is about absolute value or relative value. If it isabout relative value, you need to know what the BASE is. If comparison isinvolved, you need to find out the relationship between the bases.


这道题的答案是选C么？可是不理解B的选项的意思，B选项是把总体中个人的性质归因于平均水平么？然后答案中那个5%X<99%Y，这个式子是怎么列出来的啊啊啊啊？

lisney

是c呢，这道题我觉得可以这么理解，5%被检查出吸食可卡因的人（其实没有吸食）如果基数很大即x很大，就会比99%y真正吸食的人多，（题目也说了是randomly chosen，谁知道哪类人更多呢！！？？比如x是10000，y是10）这个时候就不是题目结论中的the vast majority of ……have used cocaine了，，，，
我语言表达不大好，，，童鞋你明白俺意思不~~