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Infection test

by nervesofsteel » Thu Jun 11, 2009 11:00 pm
For what percent of those tested for a certain infection was the test accurate; that is,positive for those who had the infection and negative for those who did not have the infection?
(1) Of those who tested positive for the infection,1/8 did not have the infection.
(2) Of those tested for the infection, 90 percent tested negative.

OA after some discussions...
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by tohellandback » Thu Jun 11, 2009 11:23 pm
IMO E

For what percent of those tested for a certain infection was the test accurate; that is,positive for those who had the infection and negative for those who did not have the infection?
(1) Of those who tested positive for the infection,1/8 did not have the infection.
(2) Of those tested for the infection, 90 percent tested negative.

1)we need information about those tested negative. INSUFF
2) we need information about what percentage or fraction of those tested got the accurate results-INSUFF

combining both,
10 percents tested positive, 90 percent negative
1/8 of those who tested positive did not have infection. but we still need more information about those who tested negative to find the overall percentage of people who got accurate results---so INSUFF
The powers of two are bloody impolite!!
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by ket » Fri Jun 12, 2009 1:00 am
IMO its E

I have attached how i arrived to this result, in this kind of problems IMHO it is best to use the matrix like the one I attached. It helps you see everything clearly.

I think it is obvious from the matrix that there is no way we can arrive to the the answer with the information given.

Looking forward to seeing OA.:)[/img]
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by nervesofsteel » Fri Jun 12, 2009 8:26 am
ket wrote:IMO its E

I have attached how i arrived to this result, in this kind of problems IMHO it is best to use the matrix like the one I attached. It helps you see everything clearly.

I think it is obvious from the matrix that there is no way we can arrive to the the answer with the information given.

Looking forward to seeing OA.:)[/img]
thanks.. but question asks for percentage of people who were tested for this infection... and from the matrix.. 70/8% is the %age of number of people who tested rightly positive out of total number of people tested for infection...
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by gauravkhare » Fri Jun 12, 2009 11:39 am
there can also be case of persons who were tested negative but were actually positive. No info is given about them. Hence E.
gaurav
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by ket » Fri Jun 12, 2009 1:06 pm
nervesofsteel wrote:
ket wrote:IMO its E

I have attached how i arrived to this result, in this kind of problems IMHO it is best to use the matrix like the one I attached. It helps you see everything clearly.

I think it is obvious from the matrix that there is no way we can arrive to the the answer with the information given.

Looking forward to seeing OA.:)[/img]
thanks.. but question asks for percentage of people who were tested for this infection... and from the matrix.. 70/8% is the %age of number of people who tested rightly positive out of total number of people tested for infection...

hm.... well I don't get you.... the question asks :
For what percent of those tested for a certain infection was the test accurate . that is,positive for those who had the infection and negative for those who did not have the infection? - i.e. both positive and negative

the matrix only gives those that were positive and test was correct , it lacks those that were negative in test and test was correct .. if we knew both of these the answer would be their sum... there is no way we can find those whose test was negative and answer was correct, hence E.
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by nervesofsteel » Sat Jun 13, 2009 12:57 am
Thanks OA is E
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by mcdesty » Sat Jul 19, 2014 9:36 pm
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by Brent@GMATPrepNow » Sat Jul 19, 2014 9:43 pm
Some nice solutions to this question here: https://www.beatthegmat.com/for-what-per ... 70596.html

Cheers,
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Brent Hanneson - Creator of GMATPrepNow.com
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by GMATinsight » Sun Jul 20, 2014 3:16 am
nervesofsteel wrote:For what percent of those tested for a certain infection was the test accurate; that is,positive for those who had the infection and negative for those who did not have the infection?
(1) Of those who tested positive for the infection,1/8 did not have the infection.
(2) Of those tested for the infection, 90 percent tested negative.

OA after some discussions...
I think the best technique in such questions is to look for what is not given by statement in order to prove that the given statement is/are INSUFFICIENT

Statement 1)Of those who tested positive for the infection,1/8 did not have the infection

This statement doesn't provide any information about the people who got negative test result so can be called INSUFFICIENT within 5-10 seconds

Statement 2)Of those tested for the infection, 90 percent tested negative

This statement doesn't provide any information about the people who got positive test result so can be called INSUFFICIENT within 5-10 seconds

Combining the two statements
Two statements together are INCOHERENT because the relationship between the number of people tested Positive and the number of people tested Negative is not provided. Therefore INSUFFICIENT

Answer: Option E within 30 seconds. :lol: :lol: :lol:
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