Critical Reasoning

Appendicitis (inflammation of the appendix) is potentially fatal; consequently, patients with symptoms strongly suggesting appendicitis almost 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 .......

Which of the following logically completes the passage?

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

OA: B, but i wanted to learn from the veterans how to remove incorrect answer choices.

richachampion wrote:Appendicitis (inflammation of the appendix) is potentially fatal; consequently, patients with symptoms strongly suggesting appendicitis almost 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 .......

Which of the following logically completes the passage?

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

Conclusion: The new test will cut down on unnecessary appendix removals without leading to fewer necessary procedures.

Premises: - In the past the surgery was performed unnecessarily 20% of the time.
- The new scan will produce errors 2% of the time

The key here is to recognize that when it comes to diagnostic tests, there are two types of errors. There are false positives, when a test incorrectly suggests that a healthy patient has a disease, and false negatives, when a test incorrectly suggests that a sick patient does not have the disease.

Notice the language shift in the premises. Historically, the procedure was performed unnecessarily 20% of the time. In other words, the false positive rate was 20%. The new test will produce errors, in general, 2% of the time. But we don't know kind of errors will comprise this 2%!

The conclusion is that the false positive rate will decrease, but we won't have to worry about not performing necessary procedures. In other words, we don't have to worry about false negatives.

B tells us, in essence, that all of the errors are false positives. Thus, we don't have to worry about false negatives - instances when a sick person doesn't get the procedure she requires.

I posted an explanation here:
https://www.beatthegmat.com/gprep-3-cr-3-t278301.html