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

Question

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

Masterscorp · Answer

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

srinjoy1990 · Answer

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.

Tx.

push12345 · Answer

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

Give kudos if it helps

Posted from my mobile device

kovid231 · Answer

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.

Q.type: Evaluate argument
POA: Read conclusion with structure in mind

Rephrase: On average, 5% of people who have not used ketamine test positive for K.
99% of people who have used K test positive.
Conclusion: In a random pool of people, if some people are shown to be positive for K then majority of those people would be users of K.

The author's conclusion is based on the data point that on average 99% percent of K users test positive.

A reasoning error in the argument is that the argument

To attempt such questions, it's better to follow a two-step approach.
On first read, take OS out. - These are questions not related to the topic.

A.attempts to infer a value judgment from purely factual premises.
Okay, at first gaze i don't understand what this means. Let's keep it for now

B.attributes to every member of the population the properties of the average member of the population.
Okay, so a discussion over average members and every member. Seems in sync with the passage. Let's keep it for now

C.fails to take into account what proportion of the population have used ketamine.
This talks about proportion. let's keep this too

D.ignores the fact that some ketamine users do not test positive.
Okay, this is not true. The author mentions that majority of people in a pool must be K users. This means he has accounted for the fact that there are some who do not test positive. - We can remove this.

E. advocates testing people for ketamine use when there is no reason to suspect that they have used ketamine.
OS - This is not the flaw of the passage. The flaw is about giving a blanket statement basis averages. Let's remove this too

Okay, so I could only remove 2 answer options. Let's look closely at the shortlisted answers

A.attempts to infer a value judgment from purely factual premises.
While the author does try to make an inference, and the premises are factual, in the sense, that they are data points. I am not sure what value judgement means here.
Let's move to the other 2

B.attributes to every member of the population the properties of the average member of the population.
Okay, I can negate this, considering the answer talks about properties of the average member. We are concerned about K users and people who haven't used K. 
We don't know what is an average member of the society. - This is OS. - Not related to the passage.

C.fails to take into account what proportion of the population have used ketamine.
Okay, this seems like the right answer. The author's conclusion is on the overall population but his premises are on K-users and non K-users. If we were told that K user make 90 percent of the overall population, we would have been able to ascertain the pop size and estimate the number of people who would be active users.

This option looks better than A, since it hits the flaw in argument precisely.

VerbalBot · Answer

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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

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