PT4.S1.Q23 - 'when 100 people who have not used cocaine are tested for...'

hyelimparkhyelimpark Core Member
edited August 2022 in Logical Reasoning 42 karma

I am struggling with this question-could someone help? I am struggling to understand the premise as well!
Thank you!

Comments

  • georgianablythe16georgianablythe16 Alum Member
    143 karma

    Hi there! I'm no expert, but I thought maybe I could help shine some clarity on this question for you as I just went through this LR section of PT4.

    The first thing that has begun to really help me with LR questions is truly understanding what the stimulus is saying. So, in simplifying the wordy stimulus, here's what I came up with:

    Premise 1: 100 non-cocaine users are tested for cocaine use. On average, 5 will test positive.

    Premise 2: 100 cocaine users are tested for cocaine use. On average, 99 will test positive.

    Conclusion: Therefore, if we randomly select a group of people, more of them will test positive for cocaine use, than those who test negative for cocaine use.

    Before I even approach the questions my brain is already sending off red alerts. This argument already seems flawed to me. Why? Because we don't know how many of the people in the randomly selected group will be cocaine users vs. non-cocaine users. We need to know this in order to place any kind of judgement on what the results will be. The conclusion COULD be true if we knew for certain that, let's say, 98/100 of the random people selected are cocaine users. Then, sure, based on percentages we could determine that more people in the group would test positive for cocaine than not. But we don't know that. It could also be true that 98/100 of the random people selected are NOT cocaine users. Then, based on percentages, more of the people would NOT test positive for cocaine use.

    With that in mind, going into the questions it's easier to pinpoint the correct answer (C). What if only 2% of the general population has ever used cocaine? It would be difficult to randomly select a group of people where the majority have used cocaine then.

    I hope that makes sense. Like I said, I'm no expert on LR and I'm still learning too! I'm trying to approach every LR stimulus with a critical eye, looking where the author is leaving something out, or making unfounded conclusions. It's really helped me. If you want any more info on resources I'm using feel free to private message!

  • hyelimparkhyelimpark Core Member
    42 karma

    thank you very much indeed-this is very helpful!!!

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