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When people who have not used ketamine, a narcotic drug, are tested

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When people who have not used ketamine, a narcotic drug, are tested  [#permalink]

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New post 25 Apr 2018, 01:43
2
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A
B
C
D
E

Difficulty:

  75% (hard)

Question Stats:

55% (01:44) correct 45% (01:51) wrong based on 111 sessions

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

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 ketamine.
D.ignores the fact that some ketamine users do not test positive.
E.advocates testing people for ketamine use when there is no reason to suspect that they have used ketamine.

Source: Crackverbal

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Re: When people who have not used ketamine, a narcotic drug, are tested  [#permalink]

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New post 25 Apr 2018, 02:36
The correct answer is C:

From the initial statement we know:

1) 99 of 100 persons who used Ketamine get detected
2) 5 of 100 persons who didn't use Ketamine get falsely detected


Now let's check two assumptions regarding the number of persons that use Ketamine.

1. Let's assume that 1 out of 100 people used Ketamine.

Because we test 100 persons we should likely get 6 detections. 5 false positives and 1 correct positive. Thus, we would have a rate of 1/6 = ~17% correct detections.

2. Let's assume now that 20 out of 100 persons use Ketamine.

Because we test 100 persons we should likely get 25 detections. 5 false positives and 20 correct positives. Thus, we would have a rate of 20/25 = 80% correct detections.


From this assumption test we can conclude that without the information what proportion of the population consumes Ketamine we can't properly determine the proportion of correct detections.

I hope that helps :-)
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Re: When people who have not used ketamine, a narcotic drug, are tested  [#permalink]

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New post 06 May 2018, 06:38
Straight C.

You need to know the weights to come to a conclusion as the one give in the argument. When we talk about weights, its really the proportion.

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Re: When people who have not used ketamine, a narcotic drug, are tested  [#permalink]

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New post 13 May 2018, 03:45
Argument takes into consideration that it doesn't matter the sample the resulted ones will always be from one who have used ketamine

But with info in argument we do have positive from people who have not used ketamine

So to state the result of positive in random sample we need to know how many have used ketamine and how many have not as result of positive cases vary in each case

C clearly states this

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Re: When people who have not used ketamine, a narcotic drug, are tested &nbs [#permalink] 13 May 2018, 03:45
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