PT59, LR1, Q20

lsatextreme
Posts: 523
Joined: Thu Sep 24, 2009 2:18 am

PT59, LR1, Q20

Postby lsatextreme » Sat Oct 02, 2010 9:38 pm

It's the one about Quality control investigators. I had a feeling this question was a popular miss so I googled it and saw explanations on various other forums of its explanation between B vs D, and as much as I understand why D is right, I can't seem to figure out why B is wrong. I think the issue stems from what I think it's saying vs what it's really saying. I keep thinking it's saying that the likelihood of choosing defective vs nondefective wasn't higher than the other, thus doesn't that essentially mean the same thing as D?

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Verisimi
Posts: 90
Joined: Thu Jul 15, 2010 2:01 am

Re: PT59, LR1, Q20

Postby Verisimi » Sat Oct 02, 2010 10:02 pm

From the way that I saw it, if the field inspectors were just as likely to choose a defective item for testing as a non-defective item, then 50% of the samples should be defective and 50% not. There is a slight equivocation in how they word B. You were looking for something which would have said that their selections were representative of the overall population, but what it really seems to be saying is that the probabilities of picking each is the same. If the sample defective rate is 20%, then the field inspectors weren't just as likely, but it does suggest that they chose items which they suspected to be defective.

lsatextreme
Posts: 523
Joined: Thu Sep 24, 2009 2:18 am

Re: PT59, LR1, Q20

Postby lsatextreme » Sat Oct 02, 2010 10:50 pm

Verisimi wrote:From the way that I saw it, if the field inspectors were just as likely to choose a defective item for testing as a non-defective item, then 50% of the samples should be defective and 50% not.


See I saw that explanation on all the other forums too but I can't seem to understand it. Shouldn't it just imply that you'd have a 50/50 chance of selecting either a defective or a non-defective? Why does B have to lead to the fact that the results then should have been 50% defective and 50% non-defective, therefore it being wrong? I guess that's what I don't understand

JJDancer
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Re: PT59, LR1, Q20

Postby JJDancer » Sat Oct 02, 2010 10:57 pm

I wouldn't be able to figure out percentages during LR (math fail) so I didn't take that approach.
I did get it down to B and D and correctly chose D.

They say that 5% defect is allowed but samples chosen by inspectors showed 20% defective.
The argument doesn't PRESUME that they were just as likely to chose defective or non-defective. - It is just presuming that the sample they got from inspectors is representative of the actual % that were defective from the supplier. What could cause a disconnect? Well if the inspectors only gave those for testing that they thought were defective...D!

(does that help?)

lsatextreme
Posts: 523
Joined: Thu Sep 24, 2009 2:18 am

Re: PT59, LR1, Q20

Postby lsatextreme » Sat Oct 02, 2010 11:07 pm

i think i'm about to get it...

thank you both for the help!




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