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# A certain airport security scanner designed to detect

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Senior Manager
Joined: 25 Jul 2009
Posts: 325
A certain airport security scanner designed to detect [#permalink]

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30 Jan 2010, 12:02
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(N/A)

Question Stats:

33% (01:42) correct 67% (01:46) wrong based on 6 sessions

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A certain airport security scanner designed to detect explosives in luggage will alert the scanner’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 scanner’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 scanner’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

OA comes later
Senior Manager
Joined: 22 Dec 2009
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30 Jan 2010, 13:22
A certain airport security scanner designed to detect explosives in luggage will alert the scanner’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 scanner’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 scanner’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

The reason or the stems talks about 1% of baggages with no explosives would give false alarm by the scanner....

But the conclusion is based on the percentage of baggages which have explosives...!

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Manager
Joined: 02 Oct 2009
Posts: 193

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30 Jan 2010, 14:46
Also want to go with E
Final conclusion seems to be confused with what 1% and 99% or alerts are...!

That said A intrigues me; since that does seem to be true...contributing the flawed logic...
Senior Manager
Status: Can't give up
Joined: 20 Dec 2009
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31 Jan 2010, 15:28
IMO A) ignores the possibility of the scanner’s failing to signal an alert when the luggage does contain an explosive = stmt fails to address the failures which could happen due to the system failing.

OA pls ?
Manager
Joined: 09 Jan 2010
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01 Feb 2010, 21:27
i would go with E
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Joined: 16 Jul 2009
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07 Jun 2010, 14:11
OA is E, but i dont catch why A is wrong.
Could anybody explain?
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Senior Manager
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07 Jun 2010, 19:49
noboru wrote:
OA is E, but i dont catch why A is wrong.
Could anybody explain?

same here..i went for A...still cant figure out why is A incorrect?
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08 Jun 2010, 14:18
gauravnagpal wrote:
noboru wrote:
OA is E, but i dont catch why A is wrong.
Could anybody explain?

same here..i went for A...still cant figure out why is A incorrect?

The question asks us to identify why the argument is flawed. Argument is states in the conclusion of the stem
"Thus in ninety-nine out of a hundred alerts explosives will actually be present."
The reasoning in the argument is flawed because the argument ignores that 99% of some number of baggages were checked and only 1% of that number of baggages were falsely identified to be with explosives. Choice D points out that flaw.
Choice A is incorrect, because statement A is irrelevant to the argument. Argument is concerned by the number of falsely identified bags not of failures by the detector to identify the explosive. Hopes it helps.
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Joined: 26 Mar 2010
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08 Jun 2010, 15:49
My take on this argument:

This argument tests the case of logical opposites. Lets take an example -If the police have a 99% probability of nabbing a thief, then there is a 1% chance that the thief would escape(or the police would not be able to nab the thief)
Thus, Prob(nabbing)+Prob(~ able to nab)=1

But not being able to nab does not necessarily equate to nabbing an innocent.

Likewise, for this example, if the argument tells us that there is a 99% chance of detecting a luggage with explosive, there is a 1% chance of not detecting a luggage with explosives(the luggage with explosives is passed through the scanner undetected).
Thus, Prob(detecting)+Prob(~detecting) = 1

But, the argument replaces the probability of failing to detect with probability of detecting an erroneous luggage.

Hence, the argument substitutes one group for a different group in the statement of a percentage

Thus, IMO E.
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Joined: 17 Feb 2010
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11 Jun 2010, 12:10
should be (E)

A is irrelevant to the argument. It is a nice trap to entice test takers by using familiar words from the argument.
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02 Jul 2010, 10:27
Out of 100 alerts that the detector sounds, 1% are erroneous alerts (bags with no explosives). So actual number of bags with no explosives will depend on the total number of bags screened. Number of bags with explosives will be likewise determined.

In its concluding part, the argument switches from talking percentages of alerts to number of alerts sounded.

They are not the same.

Please note that while the question says - the alert will beep erroneously FOR BAGS THAT DONT HAVE EXPLOSIVES (1% CASES) it does not expressly qualifies if it will MISS a beep for BAGS that DO contain explosives. I am assuming that the it will beep FOR ALL BAGS that do contain explosives PLUS it will beep additionally for 1% of bags that DO NOT CONTAIN EXPLOSIVES

Take a concrete example - which has been reverse-engineered from the conditions laid out in the question.

Assume there are a total of 298 bags. Out of these further assume that 98 have explosives.
The detector will erroneously beep for 1% of 200 bags with no explosives = 2 beeps.
remaining 98 beeps will correspond to bags with explosives.
Thus we see a case where out of 100 alerts - 98 and not 99 bags actually had explosives.

angel2009 wrote:
A certain airport security scanner designed to detect explosives in luggage will alert the scanner’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 scanner’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 scanner’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

OA comes later
Re: security threat   [#permalink] 02 Jul 2010, 10:27
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