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# A certain airport security scanner designed to detect

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Director
Joined: 20 Apr 2005
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A certain airport security scanner designed to detect [#permalink]

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20 Jun 2005, 10:32
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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

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VP
Joined: 30 Sep 2004
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Location: Germany

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20 Jun 2005, 11:54
A)...
_________________

If your mind can conceive it and your heart can believe it, have faith that you can achieve it.

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SVP
Joined: 16 Oct 2003
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20 Jun 2005, 12:17
A is my answer as well.

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Intern
Joined: 21 Jun 2005
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29 Jun 2005, 06:25
i would go in for D.

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Senior Manager
Joined: 06 Apr 2005
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Location: USA

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29 Jun 2005, 07:16
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.

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Senior Manager
Joined: 30 May 2005
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29 Jun 2005, 07:29
E is my choice. Consider the possibility that a 100,000 bags pass thru the scanner with NONE of them having explosives. The scanner will ring 100 times and be 100% wrong.

A is wrong because it directly contradicts an assertion in the statement.

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Senior Manager
Joined: 04 May 2005
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29 Jun 2005, 20:07
E for me

'99% being the probablity' does not translate into 99 out of 100.

In fact, 100 is too small a sample pool for the probablity calculation.

For example, I bought 100 lottery tickets and got one big win, that does not
mean my chance of winning big is 1%

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Director
Joined: 18 Feb 2005
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29 Jun 2005, 21:03
Agree with E

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Senior Manager
Joined: 03 Nov 2004
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03 Jul 2005, 20:19
E baby, its gotta be E // 0.01 -> error with NO explosive , but this doesnt mean that the rest 0.99 will have one, exactly as answer choice E.

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03 Jul 2005, 20:19
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# A certain airport security scanner designed to detect

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