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


requesting any of the instructor to help out with this one..I don't understand the argument over here.

krutik wrote:Appendicitis (inflammation of the appendix) is potentially fatal; consequently, patients with symptoms strongly suggesting appendicitis almost invariably have their appendices 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?

Let's look at the argument first. (I fixed it a little by adding the word invariably and by changing their appendix to their appendices.)

The prompt conveys that appendicitis can be fatal; so if someone seems to be experiencing appendicitis, as determined via symptom analysis, then almost invariably that person undergoes surgery. The thing is that in 20% of cases so diagnosed, the surgery is unnecessary.

Ok now look CAREFULLY at what is said about the new test. It is highly accurate, but it does provide misdiagnoses in 2% of cases. As a result "doctors can largely avoid unnecessary removals of the appendix without, however, performing any fewer necessary ones than before".

We know that 2% of the time the test gives incorrect results. Can we determine how they are incorrect? The answer is yes, because in the details there are clues.

Doctors "can largely avoid unnecessary removals". Ahah. So sometimes they still perform unnecessary surgery. That sounds like the result of misdiagnoses provided by the test.

On the other hand doctors using the test are not "performing any fewer necessary ones than before." Sooo, all the necessary ones are diagnosed.

So just from looking at the prompt, we can determine that the 2% error rate incorrectly indicates that surgery is necessary when it is not and that the test always correctly catches cases in which surgery is necessary.

Catching key details is so the way to get the right answers to CR questions.

Now let's check the answer choices to see which best completes the sentence.

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.

This is not connected to anything in the prompt and does not explain why the test helps doctors to largely avoid unnecessary surgery while always performing "necessary" surgery. (I personally am skeptical that even necessary surgery is always as necessary as they say.)

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

This nails it and is exactly what we figured out. The 2% error rate is low and is entirely made up of cases of people being diagnosed with appendicitis when they are not in fact experiencing it.

C. all of the patients who are diagnosed with this test as having appendicitis do, in fact,have appendicitis.

This does not go along with what we figured out and does not explain why the test always catches cases of appendicitis.

D. every patient who is diagnosed with this test as having appendicitis has more than one of the symptoms generally associated with appendicitis.

This may seem right somehow, but it is actually irrelevant. From the prompt know that someone could have the symptoms and still not in fact be experiencing appendicitis. So, that people given a diagnosis via the test are experiencing symptoms does not validate the test.

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.

This does not explain why the test rarely provides a positive diagnosis when a person is not in fact experiencing appendicitis or why the test always catches actual cases of appendicitis.

So by carefully looking at what is going on we can figure out that the correct answer is B.

The conclusion here is that if doctors use this new test for appendicitis, they can largely avoid unnecessary surgeries without performing fewer necessary surgeries than before.

The evidence is that, historically, the surgery has been performed unnecessarily about 20% of the time. and this new test will only produce two misdiagnoses for every 98 correct diagnoses.

When it comes to diagnostic tests, there are two kinds of errors. False positives: you test positive when you do not, in fact, have the disease. And false negatives: you test negative when you do have the disease. The false positive scenario leads to the unnecessary surgery, because in this instance, the patient thinks he has appendicitis, but does not. So when we say that 20% of the surgeries were unnecessary, we're talking about a high false positive rate. The new test is only incorrect 2% of the time, so it certainly seems as though it will be cutting down on false positives, and therefore unnecessary surgeries. The question then is whether the inaccurate diagnoses from the new test will result in more false negatives, and consequently, fewer necessary surgeries.

Answer choices B reads: the misdiagnoses produced by this test are always instances of attributing appendicitis to someone who does not, in fact, have it

This tell us that the misdiagnoses are all false positives. If there are no false negatives, then we don't have to worry about doctors performing fewer necessary surgeries than before.