In prenatal testing for spina bifida

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kinji@BTG: Junior | Next Rank: 30 Posts; Posts: 24; Joined: Sat Apr 23, 2011 9:43 pm; Thanked: 1 times; GMAT Score:760

by kinji@BTG » Sat May 24, 2014 6:58 am

In prenatal testing for spina bifida, a life threatening disease, a false positive result indicates that an fetus has spina bifida when, in fact, it does not; a false negative result indicates that a fetus does not have spina bifida when, in fact, it does. To detect spina bifida most accurately, physicians should use the laboratory test that has the lowest proportion of false positive results.

Which of the following, if true, gives the most support to the recommendation above?

(A)All laboratory tests to detect spina bifida have the same proportion of false negative results.
(B)The laboratory test that has the lowest proportions of false positive results causes the same minor side effects as do the other laboratory tests used to detect spina bifida.
(C)In treating spina bifida patients, it is essential to begin treatment as early as possible, since even a week of delay can result in loss of life.
(D)The proportion of inconclusive test results is equal for all laboratory tests used to detect spina bifida.
(E)The accepted treatment for spina bifida does not have damaging side effects.

[spoiler]
A
[/spoiler]

Source: Beat The GMAT — Critical Reasoning | Sat May 24, 2014 6:58 am

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by [email protected] » Sat May 24, 2014 11:35 am

Hi kinji,

This CR prompt is an example of a Strengthen question; we need to understand the author's logic and then find an answer that strengthens it.

The Facts:
-In a test for spina bifida, there are 2 types of incorrect results:
1) a false positive - the test says a fetus has SB, when it does not.
2) a false negative - the test says a fetus does not have SB, when it does.

The Conclusion:
-To detect SB MOST ACCURATELY, physicians should use the test that has the LOWEST PROPORTION of FALSE POSITIVES.

The Logic:
The SB test has TWO potential errors, but the conclusion tells us to use the test with the lowest proportion of FALSE POSITIVES. There must be some reason why THAT ERROR is more important (which is why we need to use the test that focuses on it). Maybe the false negatives aren't important or maybe they don't happen very often, but we need an answer that explains why the FALSE NEGATIVES are not a factor in making this decision.

The only answer that address this issue is A - it gives us a reason why the false negatives are not a factor.

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by GMATGuruNY » Sat May 24, 2014 1:58 pm

kinji@BTG wrote:In prenatal testing for spina bifida, a life threatening disease, a false positive result indicates that an fetus has spina bifida when, in fact, it does not; a false negative result indicates that a fetus does not have spina bifida when, in fact, it does. To detect spina bifida most accurately, physicians should use the laboratory test that has the lowest proportion of false positive results.

Which of the following, if true, gives the most support to the recommendation above?

(A)All laboratory tests to detect spina bifida have the same proportion of false negative results.
(B)The laboratory test that has the lowest proportions of false positive results causes the same minor side effects as do the other laboratory tests used to detect spina bifida.
(C)In treating spina bifida patients, it is essential to begin treatment as early as possible, since even a week of delay can result in loss of life.
(D)The proportion of inconclusive test results is equal for all laboratory tests used to detect spina bifida.
(E)The accepted treatment for spina bifida does not have damaging side effects.

This CR links the PROPORTION OF FALSE POSITIVES to ACCURATELY DETECTING SPINAL BIFIDA.
The assumption is that NO OTHER FACTORS -- particularly the PROPORTION OF FALSE NEGATIVES -- should be considered when deciding which tests to use.
Answer choice A supports this assumption:
All laboratory tests to detect spina bifida have the same proportion of false negative results.
If all tests have the SAME PROPORTION OF FALSE NEGATIVES, then the percentage of false negatives is irrelevant, supporting the conclusion that tests with the SMALLEST PROPORTION OF FALSE POSITIVES will yield the most accurate results.

The correct answer is A.

None of the other answer choices strengthens the conclusion that tests with the smallest proportions of false positives will be MORE ACCURATE than other types of tests.

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