求助GWD-21-4

Q4:

Which of the following most logically completes the passage?

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 ______.

1. the patients who are correctly diagnosed with this test as not having appendicitis invariably have medical conditions that are much less serious than appendicitis
2. the misdiagnoses produced by this test are always instances of attributing appendicitis to someone who does not, in fact, have it
3. all of the patients who are diagnosed with this test as having appendicitis do, in fact, have appendicitis
4. every patient who is diagnosed with this test as having appendicitis has more than one of the symptoms generally associated with appendicitis
5. 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

正确答案是B. 不是很明白。C为什么不对呢？ 请高手赐教。谢谢

DING

题干说能减少removal的同时必要手术的数量并未减少，就是因为新方法会把没有appendicitis的人确认为得了该病，所以医生会对这部分人进行手术

阑尾炎- 以前80% necessary，
            20% unnecessarily

新技术error只有2%，大量减少了unnecessarily，但是还是(no fewer)=more than necessary

那就是说比那80% necessary
        的多，那自然这error 2%就是: 本来unnecessarily误判成了有的了,

A.the misdiagnoses produced by this test are always instances of attributing appendicitis to someone who does not, in fact, have it   

总把who dose not判成have it！=unnecessarily误判成了有的

题干：AX发炎被称作AS.原来所有有AS症状的人都remove AX,但是其中20%没必要，也就是说这20%虽然有AS的症状但是实际上不是AS. 新方法错误率只有2%.

结论：使用新方法可以很大程度上减少不必要的removal,并且同时不减少必要的removal.

问题问Since_____，实际可以理解为问assumption。

（B)，新方法的错判总是把没得AS的人认为是得了AS，说明新方法的2%是unnecessary removal,对比旧方法20%，支持了结论largely avoid unnecessary removal。

(C)，新方法诊断为AS的人的确得了AS。这是说的是诊断正确的案例，没有涉及错判，所以我们还是不知道misdianosis是type i error还是type ii error(如果新方法把得了AS的人错判成没有得而没有手术,那么就减少了necessary removal，和结论相反。）另外可以按assumption题来加not削弱理解，(C)+not变成不是所有被诊断为AS的人都的确得了AS，那就是说有一部分人被诊断为没有AS但是实际上是AS病人，那么这部分人就是unnessary removal，但是这部分人有多少？如果>18%那么新方法显然不如旧方法，<18%新方法仍旧更有效。因为没有具体说是多少，所以没有削弱不成立，从而(C)不是assumption。