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## For what percent of those tested for a certain

tagged by: Brent@GMATPrepNow

This topic has 1 expert reply and 2 member replies
canbtg Senior | Next Rank: 100 Posts
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#### For what percent of those tested for a certain

Sat Oct 12, 2013 11:59 am
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
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.

I am having trouble understanding the above question . Could some one from expert community simply the approach .

Source - GMATPREP

OA E

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Rich.C@EMPOWERgmat.com Elite Legendary Member
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Sat Oct 12, 2013 1:33 pm
Hi canbtg,

This DS question essentially refers to 4 different types who were tested for an infection:

1) Tested positive AND were positive
2) Tested positive BUT were negative

3) Tested negative AND were negative
4) Tested negative BUT were positive

The question asks us what percent of the total were groups 1 and 3 (combined).

To properly answer this question, we would need LOTS of information.

Fact 1 gives us a breakdown of those who tested positive, but doesn't give us any sense of how many (or even what percent) Tested positive or negative. INSUFFICIENT.

Fact 2 gives us 90% tested negative, thus 10% tested positive, but doesn't give us any info on the "accuracy" of the tests. INSUFFICIENT

Combined, we're still missing the "breakdown" for those who tested negative, so there's no way to answer the question.

GMAT assassins aren't born, they're made,
Rich

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mevicks Master | Next Rank: 500 Posts
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Tue Oct 15, 2013 5:23 am
canbtg 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.

Since we are given only percentages we can assume a smart number (100)
We need to find the information given in red.

St1:

INSUFFICIENT

St2:

INSUFFICIENT

St1+St2:

INSUFFICIENT

We need more information to find the proportion of people tested negative but did not have the infection, thus the answer is E

Regards,
Vivek

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Brent@GMATPrepNow GMAT Instructor
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Tue Oct 15, 2013 6:44 am
I just want to point out that Vivek has done a perfect job employing a technique known as the Double Matrix Method. This technique can be used for most questions featuring a population in which each member has two characteristics associated with it.
Here, we have a population of patients, and the two characteristics are:
- has infection or doesn't have infection
- tested positive or tested negative

Then try these additional practice questions that can be solved using the Double Matrix Method:
- http://www.beatthegmat.com/mba/2011/05/05/random-double-matrix-question-1
- http://www.beatthegmat.com/mba/2011/05/09/random-double-matrix-question-2
- http://www.beatthegmat.com/mba/2011/05/16/random-double-matrix-question-3
- http://www.beatthegmat.com/ds-quest-t187706.html
- http://www.beatthegmat.com/overlapping-sets-questions-t183320.html
- http://www.beatthegmat.com/finance-majors-non-finance-majors-overlapping-set-question-t167425.html
- http://www.beatthegmat.com/ds-french-japanese-t222297.html
- http://www.beatthegmat.com/sets-t269449.html#692540
- http://www.beatthegmat.com/in-costume-for-halloween-t269355.html#692116

Cheers,
Brent

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