请教各位大侠，对OG 171的疑问

171. A recent report determined that although only three percent of drivers on Maryland highways equipped their vehicles with radar detectors, thirty-three percent of all vehicles ticketed for exceeding the speed limit were equipped with them. Clearly, drivers who equip their vehicles with radar detectors are more likely to exceed the speed limit regularly than are drivers who do not.

The conclusion drawn above depends on which of the following assumptions?

(A) Drivers who equip their vehicles with radar detectors are less likely to be ticketed for exceeding the speed limit than are drivers who do not.

(B) Drivers who are ticketed for exceeding the speed limit are more likely to exceed the speed limit regularly than are drivers who are not ticketed.

(C) The number of vehicles that were ticketed for exceeding the speed limit was greater than the number of vehicles that were equipped with radar detectors.

(D) Many of the vehicles that were ticketed for exceeding the speed limit were ticketed more than once in the time period covered by the report.

(E) Drivers on Maryland highways exceeded the speed limit more often than did drivers on other state highways not covered in the report.

这题真是没明白为啥选B，嘿嘿，这题，是全篇OG中，唯一一题，我觉得一点谱都没有的。

我的思路是

3% equipped

33% ticketed are equipped → equipped are more likely to exceed

第一个前提和第二个，没有啥联系啊！

是啊, 我也没弄明白

http://forum.chasedream.com/dispbbs.asp?boardID=24&ID=4303&page=1

找到以前关于这题的讨论了，我明白了，你们明白没有？

答案不讨论了，关键讨论推理过程。

装雷达/ 所有车=3% -> 不装雷达/所有车=97%

装雷达且被抓/ 被抓= 33% -> 不装雷达且被抓/ 被抓= 67%,

其中必须有一个假设,就是抓人是随机的.

否则，如果警察故意抓装雷达的,那么结论就不成立了.

如果装不装雷达都一样的话, 应该在被抓的人中，有3%是雷达车, 97%是非雷达车.

但事实不是这样.

抓到的33%是雷达车, 67%是非雷达车,说明雷达车有个什么原因造成了它在被抓的车中,发生概率较大, 而非雷达车发生概率比较小.

这个原因, 就是文中结论,说 雷达车比非雷达车容易超速.

曾经尝试无数办法,希望用数学公式推出来,都未成功. 好象这么思考最容易了。

有大牛用概率公式告诉我如何推出的话, 太感谢了 .

对于A选项,我觉得 可以改成一个非常好的weaken 题目.

Drivers who equip their vehicles with radar detectors are more likely to be ticketed for exceeding the speed limit than are drivers who do not

也就是说警察喜欢抓装雷达的,  那么就是他因削弱. 但是

Drivers who equip their vehicles with radar detectors are less likely to be ticketed for exceeding the speed limit than are drivers who do not

只能起加强作用. 因为警察不喜欢抓装雷达的, 但是雷达车还是 unusually likely 被抓, 说明

雷达车更有问题了. 本选项应该是无关.

对于假设题目的取非 来验证的,我觉得也需要慎重.

less取非变成more,但是我觉得其实是错误的. 因为还有the same likely这种可能性.

事实上,原文的假设是no more likely.

Drivers who equip their vehicles with radar detectors are more likely to be ticketed for exceeding the speed limit than are drivers who do not

我觉得ETS 很多解释并没有涉及出题本质, 他们出题的时候绝对不是解释的那么简单,而是考虑的非常严密.

最简单的理解还是：两个percentage说明了超速被开罚单比例是装雷达的高，文章给出的结论是装雷达的车更容易超速。

答案明显是将两者联系起来，超速被开罚单的人就是易超速的人。把这些概念缩小后发现这个题目很简单，关键还是被它的表述搞昏头了，两个be more likely to就晕了。

再次看到这一题，思路跟此兄一样一样的。

最后就是说肯定是装雷达与超速有关

红句不对吧？不是所有的车都会超速的啊！