Some of the wrong Kaplan explanations are really wrong

fredshen

Overall I think Kapalan is ok on the explanations. But some of the explanations are downright second guessing trying to fit the credited answer choice  that are not even funny. It bothers me.  I will post some here just to warn you. 

1.

1994.12.2.24

A certain airport scanner designed to detect explosive in the luggage will alert the scanner's operator wheneven a piece of luggage passing under the scanner containing explosive. The scanner will errornously alert the operator for only one percent of the luggage that contains no explosive. Thus in 99 out of 100 alerts explosive will actually be present.

The Kaplan explanation is :

The first sentence merely gives us set up information. The second sentence gives us the crux of the evidence, so let’s paraphrase what it says to make it more concrete: If we run one hundred clean pieces of luggage past the scanner, it will wrongly sound an alert only once. The author concludes from this that in ninety-nine out of a hundred actual alerts, explosives will be present. Did you see the scope shift? The evidence and conclusion are dealing with percentages based on two different groups: The evidence involves the percentage of clean pieces of luggage that trigger false alerts, while the conclusion deals with the percentage of alerts that are accurate (meaning that the luggage does contain explosives).

If you have difficulty seeing how they differ, think about it this way: Picture a piece of luggage without explosives, and one with explosives. Here’s the evidence: The chance of a false alert on the first clean piece is 1 in 100. From this, the author attempts to show that the chance of an accurate alert on the second explosive-containing piece is 99 in 100. This conclusion is faulty because the percentages are based on different groups.
     

Well, I think the Kaplan writer had difficulty seeing this little puzzle itself. The conclusion is not about the percentage of alerts that are accurate as Kaplan claims to be. The accuracy of detecting dirty luggage is not a concern in the stem. It is really about the percentage of false alerts. You could actually have 99% accuracy detecting dirty luggage, but that still wont show that the  in 99 out of 100 alerts explosive will actually be present.

Therefore, the key here is not that % of clean piece of luggage is substitued to show % of dirty piece of luggage. The real key is that % of clean piece of luggage is subsittued to show % of all alerts. We know that 1% of clean piece of luggage are flagged wrong, so there are some false alerts. But what's the percentage these false alerts consisting of the over all alerts? According to the claim it's 1%, which is projected from the first relationship. But without knowing the number of total alerts and number of false alerts, such percentage cannot be calculated. 

Well, I think the Kaplan writer had difficulty seeing this little puzzle itself. The conclusion is not about the percentage of alerts that are accurate as Kaplan claims to be. The accuracy of detecting dirty luggage is not a concern in the stem. It is really about the percentage of false alerts. You could actually have 99% accuracy detecting dirty luggage, but that still wont show that the  in 99 out of 100 alerts explosive will actually be present.

Therefore, the key here is not that % of clean piece of luggage is substitued to show % of dirty piece of luggage. The real key is that % of clean piece of luggage is subsittued to show % of all alerts. We know that 1% of clean piece of luggage are flagged wrong, so there are some false alerts. But what's the percentage these false alerts consisting of the over all alerts? According to the claim it's 1%, which is projected from the first relationship. But without knowing the number of total alerts and number of false alerts, such percentage cannot be calculated. 

[此贴子已经被作者于2006-11-28 4:54:31编辑过]

Lycus

总结一下楼上的发言: 这套安全扫描系统对无爆炸物品的行李作出错误判断的可能性是1%, 但对爆炸物品的准确率不知道, 所以无法判断当100个炸弹来了以后的情况. 完毕

另外, Kraplan是误人子弟

fredshen

以下是引用Lycus在2006-11-28 11:31:00的发言：

总结一下楼上的发言: 这套安全扫描系统对无爆炸物品的行李作出错误判断的可能性是1%, 但对爆炸物品的准确率不知道, 所以无法判断当100个炸弹来了以后的情况. 完毕

另外, Kraplan是误人子弟

但对爆炸物品的准确率不知道, 所以无法判断当100个炸弹来了以后的情况

No, because the number of total alerts and number of false alerts are unknown, 所以无法判断当 the percentage of false and correct alerts in total total alerts.

爆炸物品的准确率 is irrelevant in this case.

No let me take last statement back. If the acurracy is 0% in detecting real explosives, only then false alert percentage can be known. What is it?    

[此贴子已经被作者于2006-11-28 14:14:28编辑过]

fredshen

Here is another one

2001.1.25

A 1991 calculation was made to determine what, if any, additional health-care cost beyond ordinary are borne by society at large for people who live sedentary life-style. The figure reached was life time on average $1650. Thus people's voluntary choice not to excercise places a significant burden on society.

credited answer is

c) Physical conditions that eventually require medical or nursing-home care often first predispose a person to adopt a sedentary life.

Kaplan's explanation

An issue that the author fails to take into account must be relevant to the author’s scope.
“Thus” signals the conclusion: Society is burdened by people’s voluntary choice not to exercise.
The evidence is the greater cost to society of almost $2,000 additional per sedentary—i.e. not exercising,
not active—citizen. The evidence, however, never brings up the “voluntary” nature of sedentary living, and
therein lies the argument’s flaw. If (C) is true, then in fact the cause-and-effect to which the author points
is reversed. It’s not that sitting around raises the cost to society; it’s that the illnesses require one to sit
around; the lack of exercise is hardly voluntary, and the costs are run up by the greater costs of care.

Kaplan is right in there is a scope shift, but it does a real poor job explaining why C is correct. Looks like the Kaplan writer tries real hard and come up with cause effect reversal with a vicious cycle explanation to fit with C. It really doesnt make any sense whatsoever. Actually it's quite simple here. The conclusion says that people's voluntary choices of sedentary life are sucking those $1605 on average out of the society. But in fact, as C shows, the (majority) of those money have gone to seniors that are required not be active because of their physical condtions), and therefore the lazy bunch have not gotten much of those money and caused much burden to the society.  

[此贴子已经被作者于2006-11-28 14:43:08编辑过]