Tips and Tricks: Data Sufficiency | Data Insights for GMAT PDF Download

What is GMAT Data Sufficiency?

Data sufficiency questions on the GMAT are arguably the best way to evaluate your business management skills. In real-world scenarios as an investor, manager, or consultant, you often don't have all the information you need, and time and resources are limited.
The essence of the data sufficiency section is to assess if you have adequate information to make a decision (Yes/No) or answer a particular question (Value). It mirrors the practical challenge of making informed decisions with incomplete but crucial information in the business world.

Tips to Solve Data Sufficiency Questions

Data Sufficiency Tip #1

For Yes/No questions, recognize and remember that “sometimes yes” and “sometimes no” is not good enough. “Always” is mandatory for sufficiency.

• Every data sufficiency question will include two pieces of information (or Statements) that are clearly labeled (1) and (2).  
• This question type is designed to trick your process of reasoning, and there’s no shortage of options at the question writer’s disposal to accomplish their task. 
• For example, 
  consider the question: “Is Elise’s dog a puppy?” Two statements are given:
  A puppy is a dog less than or equal to 3 years of age
  Elise got her dog less than 3 years ago
  Elise may have gotten her dog when it was a puppy. However, she may also have adopted her dog when it was an adult. Thus, we don’t have sufficient information to answer this yes/no question, and we can confidently say additional information is necessary to definitely answer it.

Data Sufficiency Tip #2

Memorize the 5 Data Sufficiency Answer Choices. They are always the same, and the order never changes!

• You can remember the answer choice options (and order) through this handy data sufficiency mnemonic: (1)-(2)-(T)-(E)-(N). See the full breakdown below to understand the range of answer choices from ‘A’ through ‘E’:

Statement (1) alone is sufficient to answer the question; Statement (2) alone is not
Statement (2) alone is sufficient to answer the question; Statement (1) alone is not
Only when considered together (T) do you have sufficient information to answer the question
When considered individually, either (E) statement provides sufficient information to answer the question
When considered alone or together, neither (N) statement provides sufficient information to answer the question

Data Sufficiency Tip #3

Evaluate each of the two statements individually prior to assessing them together.

This is one of the most common ways the question writers like to trip you up. It’s critical that the only time you use all the available information from statements (1) and (2) is after you’ve fully determined they were both insufficient on their own. If you avoid this common mistake, you’ll easily do better than the bottom one-third of exam takers.
Let’s take a look at a couple of examples.
Is z >= 1/z?
(1) z is positive
(2) |z| >= 1
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.
E. Statements (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data specific to the problem are needed.
Number properties show up a lot in the GMAT quant section. The test creators enjoy challenging students with tricky questions in this area. It's crucial to understand that when they say "number," it can be various things.
Positive
Negative
Zero
Fraction
Square Root
Decimal, pi, etc.

Data Sufficiency Tip #4

Test by plugging in “numbers of all types.”

For the above question, you can rule out Statement (1) by plugging in a positive fraction.
Students who only plugged in positive integers would have incorrectly assumed Statement (1) alone was sufficient. For Statement (2), plugging in both positive and negative numbers leads to opposing conclusions, so (2) is also insufficient. Only when combining together (T) the two statements do we achieve sufficiency. Hence, Answer C.

Data Sufficiency Tip #5

Resist the urge to crunch numbers. You don't have to figure out the precise answer. Your goal is simply to determine if there's enough information to answer the question.

Some students might hurry to solve the question by creating equations for each statement. But, instead of getting caught up in solving a bunch of equations, just notice that when you put both statements together (T), you have enough info to answer the question. So, the answer is C.

Examples:

Q1: A certain straight corridor has four doors, A, B, C and D (in that order) leading off from the same side. How far apart are doors B and C?
Statement 1: The distance between doors B and D is 10 meters.
Statement 2: The distance between A and C is 12 meters.
(a) statement 1 alone is sufficient, but statement 2 alone is not sufficient to answer the question
(b) statement 2 alone is sufficient, but statement 1 alone is not sufficient to answer the question
(c) both statements taken together are sufficient to answer the question, but neither statement alone is sufficient
(d) each statement alone is sufficient
(e) statements 1 and 2 together are not sufficient, and additional data is needed to answer the question
Ans: (e)
Explanation: It is obvious that neither (1) or (2) alone can tell you how far apart B and C are, and so the answer must be C or E. To see whether putting both pieces of information together will be adequate, visualize two rods: BD of length 10 units, and AC of length 12 units. Mentally place the rods alongside each other so that C lies between B and D. Now you can mentally slide the rods past each other to see that C can lie anywhere between B and D, and so we cannot fix one value for the length BC, and the answer is E.

Q2: Two socks are to be picked at random from a drawer containing only black and white socks. What is the probability that both are white?
Statement 1: The probability of the first sock being black is 1/3.
Statement 2: There are 24 white socks in the drawer.
(a) statement 1 alone is sufficient, but statement 2 alone is not sufficient to answer the question
(b) statement 2 alone is sufficient, but statement 1 alone is not sufficient to answer the question
(c) both statements taken together are sufficient to answer the question, but neither statement alone is sufficient
(d) each statement alone is sufficient
(e) statements 1 and 2 together are not sufficient, and additional data is needed to answer the question
Ans: (c)
Explanation: From (1) we know the ratio of black socks to white, but that ratio will change when one sock is taken out. To get the new ratio, and hence the probability that the next sock will also be white, we need to know the number of socks of each type. The answer cannot be A or D. Obviously (2) on its own does not get the ratio and so B cannot be correct. But putting the information in both statements together we can solve the problem (24 white socks with a ratio of black to total of 1 : 3 means that there are 12 black and 24 white socks). The answer is C.