• Varsity Tutors
    Award-winning private GMAT tutoring
    Register now and save up to $200

    Available with Beat the GMAT members only code

    MORE DETAILS
    Varsity Tutors
  • examPAL
    Most awarded test prep in the world
    Now free for 30 days

    Available with Beat the GMAT members only code

    MORE DETAILS
    examPAL
  • PrepScholar GMAT
    5 Day FREE Trial
    Study Smarter, Not Harder

    Available with Beat the GMAT members only code

    MORE DETAILS
    PrepScholar GMAT
  • EMPOWERgmat Slider
    1 Hour Free
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    EMPOWERgmat Slider
  • Veritas Prep
    Free Veritas GMAT Class
    Experience Lesson 1 Live Free

    Available with Beat the GMAT members only code

    MORE DETAILS
    Veritas Prep
  • Kaplan Test Prep
    Free Practice Test & Review
    How would you score if you took the GMAT

    Available with Beat the GMAT members only code

    MORE DETAILS
    Kaplan Test Prep
  • Magoosh
    Magoosh
    Study with Magoosh GMAT prep

    Available with Beat the GMAT members only code

    MORE DETAILS
    Magoosh
  • e-gmat Exclusive Offer
    Get 300+ Practice Questions
    25 Video lessons and 6 Webinars for FREE

    Available with Beat the GMAT members only code

    MORE DETAILS
    e-gmat Exclusive Offer
  • Target Test Prep
    5-Day Free Trial
    5-day free, full-access trial TTP Quant

    Available with Beat the GMAT members only code

    MORE DETAILS
    Target Test Prep
  • Economist Test Prep
    Free Trial & Practice Exam
    BEAT THE GMAT EXCLUSIVE

    Available with Beat the GMAT members only code

    MORE DETAILS
    Economist Test Prep

For what percent of those tested for a certain

This topic has 2 expert replies and 1 member reply
canbtg Senior | Next Rank: 100 Posts Default Avatar
Joined
11 Oct 2013
Posted:
46 messages
Upvotes:
4

For what percent of those tested for a certain

Post Sat Oct 12, 2013 11:59 am
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

  • +1 Upvote Post
  • Quote
  • Flag

GMAT/MBA Expert

Post 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

To learn more about this technique, watch our free video: http://www.gmatprepnow.com/module/gmat-word-problems?id=919

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

_________________
Brent Hanneson – Founder of GMATPrepNow.com
Use our video course along with Beat The GMAT's free 60-Day Study Guide

Check out the online reviews of our course
Come see all of our free resources

  • +1 Upvote Post
  • Quote
  • Flag
GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMAT’s FREE 60-Day Study Guide and reach your target score in 2 months!
mevicks Master | Next Rank: 500 Posts Default Avatar
Joined
19 Sep 2013
Posted:
269 messages
Followed by:
6 members
Upvotes:
94
Post 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

  • +1 Upvote Post
  • Quote
  • Flag

GMAT/MBA Expert

Post 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.

Final Answer: E

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

_________________
Contact Rich at Rich.C@empowergmat.com

  • +1 Upvote Post
  • Quote
  • Flag

Best Conversation Starters

1 lheiannie07 112 topics
2 Roland2rule 74 topics
3 ardz24 67 topics
4 LUANDATO 53 topics
5 swerve 47 topics
See More Top Beat The GMAT Members...

Most Active Experts

1 image description GMATGuruNY

The Princeton Review Teacher

155 posts
2 image description Jeff@TargetTestPrep

Target Test Prep

106 posts
3 image description Rich.C@EMPOWERgma...

EMPOWERgmat

100 posts
4 image description Scott@TargetTestPrep

Target Test Prep

98 posts
5 image description EconomistGMATTutor

The Economist GMAT Tutor

89 posts
See More Top Beat The GMAT Experts