A certain airport security scanner designed to detect explosives in lu

Question

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

AshutoshB · Accepted Answer

IMO ANSWER E:

This is how: acc to the premise, the scanner will show 1% of false positive for all non-explosive luggage

Let's take nos: 100 non-explosive pieces of luggage, the scanner will result in 1 case of False positive but based 
 on this information, we cant deduce the accuracy for all the luggage which contain explosives as we have
 info available for the accuracy of false + for non-explosive Luggage

gilltaurus · Answer

Answer is E? Incorrectly comparing no. Of erroneous alerts with no. Of pieces of luggage with no explosives

Sent from my iPhone using  GMAT Club Forum mobile app

Abhishek009 · Answer

With the same opinion, must be (E)

Compares percentage error in one statement to the actual number of alerts !!

unraveled · Answer

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 - WRONG. There is no discussion about not alerting but only about false alert and that too when explosive is no there.

(B) draws a general conclusion about reliability on the basis of a sample that is likely to be  biased  - WRONG. There is no bias as such but a conclusion based on some fact.

(C) ignores the possibility of  human error on the part of the scanner’s operator  once the scanner has alerted him or her - WRONG. Irrelevant.

(D) fails to acknowledge the possibility that the scanner  will not be equally sensitive to all kinds of explosives  - WRONG. Not the case here.

(E)  substitutes one group for a different group  in the statement of a percentage - CORRECT. No explosive to explosive conclusion.

Answer E.

nikkimah · Answer

GMATNinja ,  karishma  - Can you explain why ans choice E is correct here? Also, since we are talking about the alerts, hence option choice A is incorrect because of that?
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

GMATNinja · Answer

Here's an example to illustrate the problem with the passage:

Let's say that there are 1,090 pieces of luggage, 90 of which contain explosives.
 
 The scanner "will alert the scanner’s operator whenever the piece of luggage passing under the scanner contains an explosive". So we'll get 90 alerts for the 90 that contain explosives. In other words, the passage does NOT say that the scanner will FAIL to alert the operator when a piece of luggage DOES contain an explosive.
 
 The passage does, however, say that the "scanner will erroneously alert the operator for only one percent of the pieces of luggage that contain no explosives". That means we should get 10 "false" alerts for the 1,000 that do not contain explosives.
 
 That's 100 alerts total, and only 90 contain explosives (not 99, as indicated in the passage).

What went wrong?

"The scanner will erroneously alert the operator for only one percent of the pieces of luggage that contain no explosives". We can only apply this percentage to luggage that does NOT contain explosives. For example, if we have 7,000 pieces of explosive-free luggage, we can expect 70 false alarms.
 
 "Thus in ninety‐nine out of a hundred alerts explosives will actually be present." The previous statement referred to one percent of CLEAN luggage. But this statement refers to one percent of ALERTS. So this statement applies the "one-percent" figure to the wrong group -- to the "alerts" group instead of to the "clean luggage" group.
 
 Choice (E) is a confusing way of saying that the "one-percent" is erroneously applied to the "alerts" group when it should only be applied to the "clean luggage" group.

I hope that helps!

VerbalBot · Answer

Automated notice from GMAT Club VerbalBot:

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

This post was generated automatically.

A certain airport security scanner designed to detect explosives in lu

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Critical Reasoning (CR) Questions

A certain airport security scanner designed to detect explosives in lu

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards