PrepTest 151, Section 3, Question 13
A new screening test has been developed for(34 more words not shown)
Which one of the following(11 more words not shown)
- AIt confuses the(24 more words not shown)
- BIt makes a(19 more words not shown)
- CIt fails to(18 more words not shown)
- DIt confuses a(30 more words not shown)
- EIt confuses the(23 more words not shown)
Reading the argument
What about false positives?
The research tells us that everyone with syndrome Q gets a positive result. It doesn't tell us what happens when someone without the syndrome takes the test. A test could catch every actual case and also label healthy people positive.
Take the extreme: imagine a useless test that gives everyone a positive result. It would satisfy the stated research finding—anyone with Q would indeed test positive—but Justine's positive result would tell us nothing about whether she has Q. We don't need to claim the actual test is that bad. The example shows what the evidence hasn't ruled out.
What we’re looking for
For the flaw, I'd look for the jump from having the syndrome to testing positive, then from testing positive back to having the syndrome. The second claim needs information the first doesn't provide.
Working the answers
- ACorrect
The premise gives us a result for people known to have syndrome Q: they test positive. The conclusion starts instead with a person known to test positive and says she must have Q. But a positive result could also come from someone without the syndrome. A names the exact switch that allows an unexcluded false positive to be treated as a certain diagnosis.
- B
We are given research supporting the claim that people with Q test positive. The conclusion isn't a new general scientific claim that the test is accurate in every respect; it's a diagnosis of Justine. The problem remains even if we fully accept the research finding, because it says nothing about false positives.
- C
A person without Q and a person who doesn't test positive needn't be the same thing. But the argument doesn't draw its conclusion by treating those two negative descriptions as interchangeable. It starts with Justine's positive result and infers the syndrome. A describes that actual direction of reasoning directly, rather than a different possible confusion about negative results.
- D
Applying a rule about a group to an individual isn't inherently a mistake. If we knew Justine had Q, the stated rule would let us infer her positive result. Here we know the result instead, so the rule doesn't establish that she belongs to the group with Q. The error is the direction of that inference, not simply moving from a general rule to one person.
- E
We haven't been told that the test has no reliable results. We have a specific reliable pattern when the syndrome is present. The argument overuses that limited information; it doesn't confuse two claims that the test is wholly unreliable for detecting presence or absence.
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