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Assumption

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Assumption

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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.

OA is a... what is the best approach to determine this answer?

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Roland2rule wrote:
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.

OA is a... what is the best approach to determine this answer?
Try negation: the correct answer, when negated, should undermine the argument.

'A' negated: Of all the drivers and front-seat passengers in the survey, NO more than 20 percent were wearing seat belts at the time of their accidents.


If no more than 20% of accident victims were wearing their seat belts, then at least 80% were not wearing their belts. Well, if 80% of the sample population didn't wear belts, then if belts confer no positive benefits at all, we'd expect 80% of any random sample not to wear belts. In other words, the fact that so many of the seriously injured didn't wear belts would not have been a function of the dangers of going beltless, but rather, a function of the disproportionate representation of non-belt-wearers in general. Because the negation of A undermines the argument, we know it's the correct answer.

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