abhi758 wrote:
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
Screening for NANB will start.
5% of prospective donors will be disqualified. (So if 100 people come to donate, only 95 will be allowed after NANB screening because 5 will have NANB).
But only 1/3rd will be caught and 2/3rd will still be missed. So these 5 make up only 1/3rd of total who carry NANB. So another 10 will be there carrying NANB but they will be missed.
Conclusion: About 10 percent of actual donors will still supply NANB-contaminated blood.
10 out of 95 allowed donors will have NANB so that's 'about 10%'. Looks right. Now the gap is will all these 95 supply blood i.e. will all of them be allowed to supply blood? For our conclusion, we are assuming that all these 95 will supply blood. That NANB test was the only hurdle they needed to cross to become actual donors.
(A) Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed.
Correct. We are assuming that no other test will disqualify many NANB carriers. What if NANB carriers usually carry other infections too which are tested for too? Then of these 10, perhaps another 8 would be disqualified because of other associated infections. So negating this breaks our conclusion.
(B) Donors carrying NANB hepatitis do not, in a large percentage of cases, develop the disease themselves at any point.
Irrelevant.
(C) The estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate.
The argument takes the 1/3rd number to be accurate.
(D) The incidence of NANB hepatitis is lower among the potential blood donors than it is in the population at large.
No such comparison in the argument.
(E) The donors who will still supply NANB-contaminated blood will donate blood at the average frequency for all donors
We are talking about the donor numbers only, not the units donated and frequency of donation by each donor.
Answer (A)