Logical Reasoning

PT151 · S3 · Q13 A new screening test

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A new screening test has been developed for syndrome Q. Research has shown that the test yields a positive for syndrome Q whenever the person tested has that syndrome.

Conclusion (so)

Justine must have syndrome Q.

Evidence (since)

Justine shows positive on this test.

Whenever a person has syndrome Q, the test shows positive.

Evaluate

Any time we're doing a Flaw questions and we see a conditional logic premise (whenever), we have a 90% or better suspicion that they're just testing their #1 famous flaw, Necessary vs. Sufficient.

The sufficient indicator whenever (just like when, if, any, each, all, every) tells us to put the clause that follows on the left of the arrow, so we get this rule:

person tested → test comes has syndrome Q out positive

We would not expect the author to either illegally reverse this rule or negate it.

REVERSAL

Bob tested positive. Thus, Bob was syndrome Q.

NEGATION

Bob doesn't have syndrome Q. Thus, the test will not come back positive.

And we see that they did indeed argue via a reversal.

Goal

Look for an answer calling out the illegal Necessary vs. Sufficient confusion.

Or look for an answer making the same objection more conversationally: just because Justine tested positive doesn't guarantee she has Q. Tests sometimes provide false positives, where you think you're pregnant but you're not. You think you have covid, but you don't.

This test apparently does not have false negatives. If you have Q, the test will say positive. But it's possible this test gives out false positives. You don't have Q, but the test says positive anyway.

13.

Which one of the following most accurately describes a flaw in the reasoning in the argument?

  1. Correct

    It confuses the claim that

    Why this is right

    This describes the Necessary vs. Sufficient error by comparing the conditional the author actually gave us with the conditional the author would have needed to give us in order to reach her conclusion. She gave us, "when syndrome is present, test is positive" Q is present → test is positive She wanted a claim that says "any subject who tests positive has the syndrome" test is positive → Q is present After all, she tells us that Justine has tested positive. That doesn't trigger anything in the first rule, the one the author presented. Another way to say all this is, "the author confuses a rule where testing positive is the Necessary condition with a rule where testing positive is the Sufficient condition". But it triggers the 2nd rule and would allow the author to reach her conclusion

    Skill tested: Flaw · how this choice captures the argument's function is the move to repeat next time.

    90% picked this

  2. It makes a general claim

    Out of Scope: general accuracy claim

    There is no explicit claim made regarding the accuracy of the test, so this answer is just descriptively false. The author is implicitly accepting the accuracy of the test, that's true. She does say that "research has shown ..." that the test yields no false negatives. So there also kind of is some scientific justification to her trust the accuracy of "if you have syndrome, then test will say positive".

    3% picked this

  3. It fails to adequately distinguish

    Scrambled Ideas

    This has an overall form that could work if we inserted the right ideas into it: This argument failed to distinguish between testing positive for Q and actually having syndrome Q (i.e. it failed to appreciate the possibility of false positives -- a positive test doesn't mean for sure you have it). This answer is saying the author failed to appreciate the possibility of false negatives --- that you might not test positive but still actually have syndrome Q.

    4% picked this

  4. It confuses a claim about

    Out of Scope: group vs. individual

    There's no distinction anywhere in this argument between the test being accurate for groups but not for individuals, or vice versa. This answer seems to be teasing some sort of Sampling or Part vs. Whole issue that wasn't present in the argument. Like, "even though that was true for this group, it won't necessarily be true for Justine". We can't match that up with this argument.

    2% picked this

  5. It confuses the test’s having

    Out of Scope: no reliable results

    When a Flaw answer choice is saying the author confused X with Y, it's saying that X is what was talked about in the evidence but Y is what the author was apparently thinking in drawing her conclusion. Neither the evidence nor the conclusion talked about "a test having no reliable results".

    1% picked this

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