#13 - A new screening test has been developed for syndrome Q

[Administrator]

Please post your questions below!

[fendrick]

I had trouble diagramming this question, though i was still able to land on the correct answer. Can you please give a breakdown?

[Jay Donnell]

Hi Fendrick!

This is yet another example of a Mistaken Reversal being the culprit behind a Flaw question, albeit not a particularly obvious one!

Using the key term of whenever as a sufficient indicator (same as 'if'!), the premise should read:

Person tested has the syndrome ----> positive test result for Syndrome Q.

That basically means that every patient who actually has the condition is accurately notified by the test. However, maybe the test is so sensitive that every single person who takes the test comes up positive for Syndrome Q, even if they don't actually have the syndrome?

This is the key to the flaw between the next premise and the conclusion.

Justine showed a positive test result, so the argument concluded that she must in fact have Syndrome Q.

Conditionally, the argument can be seen as such:

(PHS = patient has the syndrome)
(PTR = positive test result)

PHS --> PTR
PTR
_________
PHS


The reasoning used by the argument involves a principle that implies: (PTR-->PHS), when the actual premise originally
stated: (PHS --> PTR).

This Mistaken Reversal is represented in answer choice A when it says: "It confuses the claim that a subject will test positive when the syndrome is present with the claim that any subject who tests positive has the syndrome."

Realizing that both 'when' and 'any' serve as sufficient indicators (same as if!), that is the verbal description of what the conditional mix up was: PHS-->PTR confused as meaning PTR-->PHS.

I hope that was helpful!

-Jay