1000 CR test 2 question 19

magical cook · Posted: Fri Mar 30, 2007 2:06 pm Post subject: 1000 CR test 2 question 19

Hi,

I thought the answer is B) (cause it focuses on NANB disease rather than other infections) ;However, the correct answer turn out to be A)

Do you know why A is better?

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.

rajesh_ctm · Posted: Sat Mar 31, 2007 7:48 am Post subject:

First, let us try to understand what the argument is.

Blood banks will shortly start to screen all donors for NANB hepatitis.
Ok.

The new screening tests are estimated to disqualify up to 5 percent of all prospective blood donors.
So out of 100 prospective donors, 5 are estimated to be screened.

The new tests will miss two-thirds of donors carrying NANB hepatitis.
So, if they are catching five, they are not able to catch 10 others.

Therefore, about 10 percent of actual donors will still supply NANB-contaminated blood.
We already saw that 10 out of our 100 have NANB but they are not caught. So far so good.

Now let us see which of the given options are assumptions in this calculation.

(A) Donors carrying NANB hepatitis do not, in a large percentage of cases, carry other infections for which reliable screening tests are routinely performed.
Well, this is definitely an assumption. If a large percentage of people having NANB also carry other diseases, a large percentage of our 10 people are caught, so our number (10%) does not hold good.

(B) Donors carrying NANB hepatitis do not, in a large percentage of cases, develop the disease themselves at any point.
At any point they may develop the disease or may not. How does it affect our calculation? It doesn't.

(C) The estimate of the number of donors who would be disqualified by tests for NANB hepatitis is an underestimate.
Come on, all calculations are based on assumption that estimates are correct. Does any one ever assume that the underlying estimations are incorrect?

(D) The incidence of NANB hepatitis is lower among the potential blood donors than it is in the population at large.
Never assumed this! We never referred to the population at large while coming up with 10% number.

(E) The donors who will still supply NANB-contaminated blood will donate blood at the average frequency for all donors.
Frequency of donations, out of scope!

magical cook · Posted: Sat Mar 31, 2007 10:26 am Post subject:

Thanks rajesh for the explanation.

It's more clear that A) is better. But just FYI, my understanding for B) originally was "they may develop NANB hepatitise ven after screen was done and the result was negative." But I think the assumption is too far...