Friday, May 23, 2008

How to conquer Data Sufficiency (DS) on GMAT

DATA SUFFICIENCY (DS)

Data Sufficiency (DS) is a part of the quantitative section on the GMAT exam. As the name suggests you have to find out whether the given data is sufficient to answer the question or no. The data sufficiency question in GMAT exam is slightly different from the regular CAT data sufficiency question. It is comparatively more tricky and forms a major part of the quantitative section on GMAT.

The data sufficiency (DS) problem consists of a question and two statements, labeled (1) and (2), in which certain data are given. You have to decide whether the data given in the statements are sufficient for answering the question. Using the data given in the statements plus your
knowledge of mathematics and everyday facts, you must indicate whether
A. statement (1) ALONE is sufficient, but statement (2) alone is not sufficient to answer the
question asked;
B. statement (2) ALONE is sufficient, but statement (1) alone is not sufficient to answer the
question asked;
C. BOTH statements (1) and (2) TOGETHER are sufficient to answer the question asked, but
NEITHER statement ALONE is sufficient;
D. EACH statement ALONE is sufficient to answer the question asked;
E. BOTH statements (1) and (2) TOGETHER are not sufficient to answer the question asked;

The easiest way to solve this question and to avoid any confusion is to take one statement at a time. There are two parts to this questions. One is the data that is already given and the other partial data given in statements (1) and (2).

Now you have to imagine that say statement (2) does not exist and only statement (1) does. Will you able to solve the given problem now? Let me give you an example here :

1. At a certain picnic, each of the guests was served either a single scoop or a double scoop of ice cream. How many of the guests were served a double scoop of ice cream?

(1) At the picnic, 60 percent of the guests were served a double scoop of ice scream.

(2) A total of 120 scoops of ice cream were served to all the guests at the picnic.

The question asks you how many guests got a double scoop. The information you already have is that there were only two type of scoops : double and single.
Now just cover up statement (2) and see if the information in (1) is enough to answer this question. You can see that it gives only a percentage and total number of people is unknown. Hence (1) is not sufficient to answer this question.

For the same example now cover up statement (1) and see if you can answer the question. Again you can easily see that (2) gives only the total number of ice creams served. Therefore (2) also is not sufficient to answer this question.

Now see if you can answer the question using both (1) and (2). Viola!!! you can.
(1) gives you the percentage being served one kind of ice cream and (2) gives you the total number of ice creams. Using these two you can find the number of ice creams of both kinds and answer the question.

Therefore, your answer would be C.

Remember you are not supposed to find the answer in DS questions. Hence don't go about solving the whole problem and waste your time. The key is to just plug in the data given and see if you can solve for a single variable. To achieve sufficiency, there must be as many equations as there are variables.

TIPS and TRICKS for DS :

  • In Data Sufficiency questions, you are usually being asked 1 of 3 things:
  1. A specific value - like in the example above
  2. A range of numbers
  3. Yes/No or True/False whatever you may like to call it
  • Start with each statement separately to see the sufficiency and if they do not satisfy the question then look at both the statements together.
  • Eliminate wherever possible. For Instance if (1) is not sufficient A and D cannot be the answers so you can do better guesswork out of BCE.
  • On harder DS questions, answer choices tend to be more sufficient than they might appear.
  • DON’T CHOOSE (E) if you have to guess.
  • Pick between (A) or (C), if you can eliminate (B).
  • Historically, (A) is slightly more common as the right answer.
  • About 1/3 of DS questions are YES/NO questions.
  • Only about half the time do you have to look at both statements in combination.
  • Do not make unwarranted assumptions. For instance for the following question :
Did Person A get over 50% of the vote?

(1) Person C got 49% of the vote.
(2) Person A got 25,000 of the 100,000 votes cast.

Most people would assume here that there are only 2 persons A and C and take (1) as sufficient. Be very careful with such type of questions and do not make this mistake of assuming the data not given.

Another common mistake people make is assuming the information given in statement (1) in solving the problem with statement (2). This is a very common exam trick and most people fall for it atleast in their initial study period. Hence view statements separately first and only when you are unable to answer the question this way take both the statements together.


On YES/NO DS questions, if a statement answers the
question conclusively in the affirmative or in the
negative, then IT IS SUFFICIENT.

Example :
Is it true that a>b?
(1) 2a > 2b
(2) a + c > b + c

You can see that both (1) and (2) can answer this question independently. Hence you should not care about any real values here. I am assuming you would be able to solve this on your own and hence not putting the explanation here.

These tricks really helped me in scoring well on DS and I hope they would be useful for you too.

To download problems for DS you can go to the download section of the blog. It contains more than enough for you to get good practice on this section! All problems come with detailed solutions and in case you find any difficulty understanding any problem you can always ping me!

Currently the download section is not ready but if you need the material immediately you can ping me again!

3 Comments:

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nancy john said...

Good post about GMAT it is very useful for students

GMAT Data Sufficiency

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