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A certain airport security scanner designed to detect [#permalink]
20 Jun 2005, 11:32
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24. A certain airport security scanner designed to detect explosives in luggage will alert the scannerâ€™s operator whenever the piece of luggage passing under the scanner contains an explosive. The scanner will erroneously alert the operator for only one percent of the pieces of luggage that contain no explosives. Thus in ninety-nine out of a hundred alerts explosives will actually be present.
The reasoning in the argument is flawed because the argument
(A) ignores the possibility of the scannerâ€™s failing to signal an alert when the luggage does contain an explosive
(B) draws a general conclusion about reliability on the basis of a sample that is likely to be biased
(C) ignores the possibility of human error on the part of the scannerâ€™s operator once the scanner has alerted him or her
(D) fails to acknowledge the possibility that the scanner will not be equally sensitive to all kinds of explosives
(E) substitutes one group for a different group in the statement of a percentage
Wow.. I am gonna digress from the crowd for a change. My answer is E. Here is why
Lets assume for argument's sake that the machine erroneouly alerted the scanner for explosives when there were none present in 10 out of a thousand cases.. or 1% of all bags. if you were to randomly choose a 100 cases, then it is quite possible that three of those 10 cases (where the machine tagged the baggage by mistake) ended up in the set of 100. Thus the conclusion drawn is wrong, and only E points the flaw in the conclusion.