confusing CR - question

[saurabh2525_gupta]

Blood banks will shortly start to screen all donors for NANB hepatitis. Although the new screening tests are estimated to disqualify up to 5 percent of all prospective blood donors, they will still miss two-thirds of donors carrying NANB hepatitis. Therefore, about 10 percent of actual donors will still supply NANB-contaminated blood.

The argument above depends on which of the following assumptions?

(A) Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed.
(B) Donors carrying NANB hepatitis do not, in a large percentage of cases, develop the disease themselves at any point.
(C) The estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate.
(D) The incidence of NANB hepatitis is lower among the potential blood donors than it is in the population at large.
(E) The donors who will still supply NANB-contaminated blood will donate blood at the average frequency for all donors

Hey guys, please help me out, I am stuck on this one!!!!!

[shovan85]

IMO A

If the donors with NANB cannot be screened for any other infections (which can be screened) then only they will accumulate to 10%. If they contain other infections and screened then our remaining 2/3 (10%) value will not hold as it will decrease.

[Brian@VeritasPrep]

Hey Shovan,

I agree - nice work! The conclusion here is faulty because we go from:

-THIS TEST will only catch about 1/3 of NANB-positive donors
-Therefore, the 2/3 of remaining donors (about 10% of the entire pool) will still supply NANB-contaminated blood

The logical flaw is that we're only using one data point - THIS TEST - to draw that conclusion. But we don't know about any other tests. There could just as likely be other tests that catch NANB symptoms and disqualify them, or those with NANB may be prone to carrying other diseases that are caught.

If we negate choice A - so we check to see what the argument would look like without it - it becomes:

A-negated: Donors carrying NANB DO, in a large % of cases, carry other infections that will be caught.

If that's the case, we can be reasonably certain that we'll catch many other NANB-infected donors, just through other channels, and the conclusion is invalid.


So...if we can prove by negating A that we do, in fact, need it to be true in order for the conclusion to hold, then A is the correct answer.





Now, I also looked long and hard at E, because if the opposite of E is true then:

The donors with NANB WILL NOT donate blood at the average frequency for all donors.

Well, to me that means that it will change the amount of blood that has NANB away from that 10%, right? Either if they donate more or less frequently, it still changes that statistic...


BUT - here's where E is DEFINITELY wrong: The conclusion is that:

10% of DONORS (not of donations) will still have the virus.

We don't care about how frequently they donate, just how many donors have it, and so E is very subtly out of scope of the conclusion.