GWD-15中的一道CR，求NN讲解！！！

mygg_1314

题目如下：

Appendicitis (inflammation of the appendix) is potentially fatal; consequently, patients with symptoms strongly suggesting appendicitis almost always have their appendix removed.  The appropriate surgery is low-risk but performed unnecessarily in about 20 percent of all cases.  A newly developed internal scan for appendicitis is highly accurate, producing two misdiagnoses for every 98 correct diagnoses.  Clearly, using this test, doctors can largely avoid unnecessary removals of the appendix without, however, performing any fewer necessary ones than before, since ______.

A  the patients who are correctly diagnosed with this test as not having appendicitis invariably have medical conditions that are much less serious than appendicitis
B  the misdiagnoses produced by this test are always instances of attributing appendicitis to someone who does not, in fact, have it  
C all of the patients who are diagnosed with this test as having appendicitis do, in fact, have appendicitis
D every patient who is diagnosed with this test as having appendicitis has more than one of the symptoms generally associated with appendicitis
E the only patients who are misdiagnosed using this test are patients who lack one or more of the symptoms that are generally associated with appendicitis
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正确答案是B.
我通过排除法做出来了，但是仔细看B又觉得讲不通。
原文中的unnecessary removal 和 necessary ones 到底是什么意思？

求大神讲解！！！

liwen0625

我来试着解释一下，这个题目太绕了。有阑尾症状的人原来都是要取出阑尾(假设一共有100人要取出阑尾)，那么有20个人误诊（他们本该不取阑尾）。现在有了test，那么100个人里只有2个人误诊。（误诊的意思有两种：1.本不用取出的人test后要取出阑尾；2.本要取出的人test后不用取出阑尾).现在说医生使用这个test很大程度上减少了不需要取出阑尾的，但是需要取出阑尾的人数并没有减少。那么问题就出在需要取出阑尾的人数上面。怎么解释这个现象呢？肯定实在误诊的人数上面下手。那就是误诊的第一条，：1.本不用取出的人test后要取出阑尾，那么就增加了取出阑尾的数量。因为误诊的数量是少数，大部分都是正确的诊断，那么不需取出阑尾的人数肯定大大增加，就avoid unnecessary的数量。

云游

Let me try this way:

This is an ASSUMPTION question, in which we are asked to give the logical  connections between premise and conclusion.  Here we proceed to pinpoint the conclusion and premise in the stimulus:

Conclusion: Conducting the test, doctors can largely avoid the unnecessary surgeries without conducting any fewer necessary surgeries.

Premise: The test can accurately diagnose who really need this surgery . The accurate diagnose rate is 98 out every 100 instances.

Gap: the 2 misdiagnosed instances.

Prephrase: What if the 2 misdiagnosed instances really needs necessary surgery?! So assume that the right answer must eliminate this possibility.

B is the right answer. If you are indecisive on B, we could try to negate this statement and see whether the negated statement will weaken the conclusion or not.

If we negated statement in choice B we will get "the misdiagnosis sometimes include cases in which patients actually have appendicitis". If this is the case, then some necessary surgeries have been missed and hence doctors' goal of "without performing any fewer surgeries" won't be achieved. Therefore, the negate choice B will undermine the conclusion. In other words, B is the assumption to make the conclusion achieved.

Personal POV, open to discussion.

兰心寒

unnecessary removal：不需要去掉appendix的； necessary ones = necessary removal ：需要去掉appendix
P：原来病人appendix一发炎就被去掉，所以20%的被去掉是unnecessary的
   新的internal scan可以使100个diagnose里面只有2个misdiagnose，2%是misdiagnose的
C：这个新方法，使得unnecessary的数量减少，necessary的数量不被不减少

gap：misdiagnose的cases必须是unnecessary的，才能满足necessary的不被减少。所以我们必须关注含misdiagnose的选项

zhouwenbin

let try to explain in another way
这题干中提到了一种变化，旧方法变到新方法，旧方法中20%的不需要被切阑尾的人被切，也就是不必要的移除，新方法有2%（大概）的误诊，结果为，不必要的移除大大减少了（20%这个数据减少，这也是要产生新方法的原因，这个20%太大了），但是，有必要的移除没有减少。也就是说假设是，这个改变过程没有使有必要的移除减少。b中正好提到这个：误诊只是那些没有必要的移除。