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A recent survey of all auto accident victims in Dole County [#permalink]
04 Jul 2005, 04:33
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A recent survey of all auto accident victims in Dole County found that, of the severely injured drivers and front-seat passengers, 80 percent were not wearing seat belts at the time of their accidents. This indicates that, by wearing seat belts, drivers and front-seat passengers can greatly reduce their risk of being severely injured if they are in an auto accident.
The conclusion above is not properly drawn unless which of the following is true?
(A) Of all the drivers and front-seat passengers in the survey, more than 20 percent were wearing seat belts at the time of their accidents.
(B)Considerably more than 20 percent of drivers and front-seat passengers in Dole County always wear seat belts when traveling by car.
(C) More drivers and front-seat passengers in the survey than rear-seat passengers were very severely injured.
(D) More than half of the drivers and front-seat passengers in the survey were not wearing seat belts at the time of their accidents.
(E) Most of the auto accidents reported to police in Dole County do not involve any serious injury.
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.
This was a very difficult question for me, and needed to put my thoughts on paper. For the first problem it is not very clear to me why A is wrong. After starting to write the explanation it kinda was clearing up.
Here's my reasoning:
Statement B says that considerably more than 20% of all drivers (survey + non-survey) wear seatbelts. This choice would have been better if they said 50% of the population wore seatbelts. That means in a population of 1000 people, 500 wore seatbelts and 500 did-not. Of this say, 100 got severly injured and according to the survey 80 of them did-not wear seatbelts and 20 of them wore seatbelts. This clearly shows that seatbelts helped. Now, I assumed the "considerably more than 20%" to be a lot more than 20% and around 50%.
Statement A says that there were more than 20% of the people in the survey were wearing seatbelts. This is a very promising choice. Remember the survey was done on all injured people, and severly injured is only a subset. So, in our number examples, this means, say 100+x got injured (surveyed) of which 100 were severly injured. Now, if 20 people wore seatbelts and there was no one in x that had seatbelts, means that everyone who wore a seatbelt got severly injured - goes against the conclusion. But the statement does say more than 20% of surveyed (100+x) wore seatbelts. However, the missing information is how many more. if there was only 1 more, it doesn't help. It needs to be comparable to the number of people not wearing seat-belts.
I think this question is rather difficult because of the ambiguity. Correct me if I am wrong.