When 100 people who have not used cocaine are tested for : GMAT Critical Reasoning (CR)
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# When 100 people who have not used cocaine are tested for

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When 100 people who have not used cocaine are tested for [#permalink]

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01 Mar 2009, 21:42
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When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument

(A) attempts to infer a value judgment from purely factual premises
(B) attributes to every member of the population the properties of the average member of the population
(C) fails to take into account what proportion of the population have used cocaine
(D) ignores the fact that some cocaine users do not test positive
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine.
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Re: CR-Cocaine Test [#permalink]

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01 Mar 2009, 21:51
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nitya34 wrote:
When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument

(A) attempts to infer a value judgment from purely factual premises
What value judgment?
(B) attributes to every member of the population the properties of the average member of the population
Yes
(C) fails to take into account what proportion of the population have used cocaine
We are not concerned about the population. Instead we are concerned about the "randomly chosen group of people"
(D) ignores the fact that some cocaine users do not test positive
It does not ignore this
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine.
Out of scope.

With Flaw questions always try to identify the flaw before moving to the answer choices. Here we are told because an average # of a particular group tests positive that the "vast majority of those who test positive will be people who have used cocaine". This is similar to saying since the average GPA in a 3rd year engineer students is a 3.0, the "vast majority" of engineers' GPA is a 3.0. What if 1st year engineer students average a 2.0? or a 4.0? You cannot make a conclusion about the average GPA of all engineer students based on a subset of the class.
This is nicely stated in B.
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Re: CR-Cocaine Test [#permalink]

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01 Mar 2009, 23:56
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When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.

A reasoning error in the argument is that the argument

Explanation:
-----------------------
(A) attempts to infer a value judgment from purely factual premises ---> In that case, it would have strengthen the argument.

(B) attributes to every member of the population the properties of the average member of the population ---> At the most, this option might strengthen the argument. If everyone shares the same property, the conclusion will strengthen. So, discard it.

(C) fails to take into account what proportion of the population have used cocaine
---> This looks fine.

The passage discusses about two groups:
Group 1. One in which, on an average, only 5 test positive
Group 2. Other in which 99 test positive.

Conclusion makes a reasoning error in assuming that even if a randomly chosen group is tested, majority of them will be the ones who have used cocaine i.e., they will belong to group 2. This may not necessarily be true.

What if the majority of the randomly chosen group comprises people belonging to group 1? Though they will still test positive, but they will belong to the group that doesn’t use cocaine. In that case, the argument will become weak.

(D) ignores the fact that some cocaine users do not test positive ---> Irrelevant

(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine. ---> Irrelevant.
---------------------

My choice is C.

Hope that helps.

Regards,
Technext
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Re: CR-Cocaine Test [#permalink]

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02 Mar 2009, 00:42
nitya34 wrote:
When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument

(A) attempts to infer a value judgment from purely factual premises
(B) attributes to every member of the population the properties of the average member of the population
(C) fails to take into account what proportion of the population have used cocaine
(D) ignores the fact that some cocaine users do not test positive
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine.

I think it's D.

In C, even if most of the users are included in the group, we can't be too sure whether majority of them will test positive.
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Re: CR-Cocaine Test [#permalink]

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02 Mar 2009, 01:34
(A) attempts to infer a value judgment from purely factual premises
-IMO
(B) attributes to every member of the population the properties of the average member of the population
- “properties of average member” is not given.
(C) fails to take into account what proportion of the population have used cocaine
- Do we really need to know “proportion” of people who took cocaine? This is a study result and mostly concluded the result from factual premises.
(D) ignores the fact that some cocaine users do not test positive
- Never ignored such case. 1% is negative
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine.
- “Reason of suspect” is OOS
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Re: CR-Cocaine Test [#permalink]

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02 Mar 2009, 01:35
I picked up B
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Re: CR-Cocaine Test [#permalink]

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02 Mar 2009, 01:48
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Now only option E is left.

Any takers?
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Re: CR-Cocaine Test [#permalink]

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02 Mar 2009, 01:55
+1 to you
OA-C

Technext wrote:
When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.

A reasoning error in the argument is that the argument

Explanation:
-----------------------
(A) attempts to infer a value judgment from purely factual premises ---> In that case, it would have strengthen the argument.

(B) attributes to every member of the population the properties of the average member of the population ---> At the most, this option might strengthen the argument. If everyone shares the same property, the conclusion will strengthen. So, discard it.

(C) fails to take into account what proportion of the population have used cocaine
---> This looks fine.

The passage discusses about two groups:
Group 1. One in which, on an average, only 5 test positive
Group 2. Other in which 99 test positive.

Conclusion makes a reasoning error in assuming that even if a randomly chosen group is tested, majority of them will be the ones who have used cocaine i.e., they will belong to group 2. This may not necessarily be true.

What if the majority of the randomly chosen group comprises people belonging to group 1? Though they will still test positive, but they will belong to the group that doesn’t use cocaine. In that case, the argument will become weak.

(D) ignores the fact that some cocaine users do not test positive ---> Irrelevant

(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine. ---> Irrelevant.
---------------------

My choice is C.

Hope that helps.

Regards,
Technext

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Re: CR-Cocaine Test [#permalink]

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06 Mar 2009, 23:29
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I've seen a few similar questions about 'false positives' and 'false negatives'. You can imagine the following situation:

100 people in Country X use cocaine
1,000,000 people in Country X do not use cocaine

Test all of these people, and 99 of the 100 cocaine users will test positive, while 50,000 of the non-users will test positive. You have 50,099 people in total who test positive, but of those, only 99/50,099 ~ 0.2 % are actually cocaine users.

Without knowing something about cocaine use in the population as a whole, we can't say very much about the people who will test positive. C is certainly the answer.
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Re: CR-Cocaine Test [#permalink]

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26 Oct 2009, 19:17
(C) fails to take into account what proportion of the population have used cocaine
Of the people who do not use cocaine, the false result is 5% whereas the false result of the people who use cocaine is only 1%. If most of the people who were survey are the people who do not use cocaine then the result cant not be trusted
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Re: CR-Cocaine Test [#permalink]

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26 Oct 2009, 23:17
Nice question. C it is ..
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Re: CR-Cocaine Test [#permalink]

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27 Oct 2009, 12:08
nice question. agree with C
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Re: CR-Cocaine Test [#permalink]

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30 Oct 2013, 21:05
Technext wrote:
When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.

A reasoning error in the argument is that the argument

Explanation:
-----------------------
(A) attempts to infer a value judgment from purely factual premises ---> In that case, it would have strengthen the argument.

(B) attributes to every member of the population the properties of the average member of the population ---> At the most, this option might strengthen the argument. If everyone shares the same property, the conclusion will strengthen. So, discard it.

(C) fails to take into account what proportion of the population have used cocaine
---> This looks fine.

The passage discusses about two groups:
Group 1. One in which, on an average, only 5 test positive
Group 2. Other in which 99 test positive.

Conclusion makes a reasoning error in assuming that even if a randomly chosen group is tested, majority of them will be the ones who have used cocaine i.e., they will belong to group 2. This may not necessarily be true.

What if the majority of the randomly chosen group comprises people belonging to group 1? Though they will still test positive, but they will belong to the group that doesn’t use cocaine. In that case, the argument will become weak.

(D) ignores the fact that some cocaine users do not test positive ---> Irrelevant

(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine. ---> Irrelevant.
---------------------

My choice is C.

Hope that helps.
is that

Regards,
Technext

The OA may be C. I think you have the reasoning wrong. The argument says that of all the people tested randomly, "those who tested positive were cocaine users" what ur saying is that " of all the people tested a majority of them are cocaine users". Pls correct me if I am wrong. I still don't see why C is the answer
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Re: When 100 people who have not used cocaine are tested for [#permalink]

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01 Jan 2014, 14:04
nitya34 wrote:
When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument

(A) attempts to infer a value judgment from purely factual premises
(B) attributes to every member of the population the properties of the average member of the population
(C) fails to take into account what proportion of the population have used cocaine
(D) ignores the fact that some cocaine users do not test positive
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine.

Hi,

For me it is clearly C.

Why? Well because if 99.99999% Of 1000000 of a population never took cocaine but only 100 has. Than (1000000*0.05 = 50000 who did not take drug and you have on the other side 99 who take drug)

Than with this demonstration, the argument is totaly flawed.

You need to take into account the proportion or the part of the population actually taking drugs.

Answer C.
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Re: When 100 people who have not used cocaine are tested for [#permalink]

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02 Jan 2014, 23:44
I choose C.We clearly need to find out the proportion of people who used cocaine in order to infer whether 'the vast majority' actually used it or not.

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Re: When 100 people who have not used cocaine are tested for [#permalink]

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03 Jan 2014, 22:19
nitya34 wrote:
When 100 people who have not used cocaine are tested for cocaine use, on average only 5 will test positive. By contrast, of every 100 people who have used cocaine 99 will test positive. Thus, when a randomly chosen group of people is tested for cocaine use, the vast majority of those who test positive will be people who have used cocaine.
A reasoning error in the argument is that the argument

(A) attempts to infer a value judgment from purely factual premises
(B) attributes to every member of the population the properties of the average member of the population
(C) fails to take into account what proportion of the population have used cocaine
(D) ignores the fact that some cocaine users do not test positive
(E) advocates testing people for cocaine use when there is no reason to suspect that they have used cocaine.

Negate the conclusion:

It is: The vast majority of those who test positive out of a random group of people would not have used cocaine.

Since this a find the flaw question we have to select a choice which can weaken the conclusion .

So let us consider this as a weaken question first and so see which choice offers the best explanation for the negated conclusion.

Choice C because a particular case of it which is , a small proportion of the population take cocaine, offers the best explanation for the negated conclusion.

This is because assume 10000 people are tested . A small proportion of that population be 1% i.e, 100 people take cocaine. 99 of them would test positive. Of the remaining 9900 people 5%, i.e., 445 would test positive. So we see the majority of those who test positive are those who had not used cocaine.
i.e, if
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Re: When 100 people who have not used cocaine are tested for [#permalink]

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