Study guide
Data Sufficiency is the GMAT's most distinctive question type, and on the current exam it lives inside the Data Insights section rather than Quantitative Reasoning, though it still draws on the same arithmetic and algebra content. A Data Sufficiency question gives you a question stem and two numbered statements, and instead of solving for a numeric answer, you judge whether each statement, alone or combined, provides enough information to answer the stem definitively. This chapter walks through the standard five-option answer format and the classic logic traps that make Data Sufficiency uniquely challenging.
The Standard Answer Format and Basic Method
Every Data Sufficiency question uses the same five fixed answer choices, so memorizing them cold is essential: (A) Statement 1 alone is sufficient, but Statement 2 alone is not sufficient; (B) Statement 2 alone is sufficient, but Statement 1 alone is not sufficient; (C) Both statements together are sufficient, but neither statement alone is sufficient; (D) Each statement alone is sufficient; (E) The two statements together are still not sufficient. The efficient method is to evaluate Statement 1 by itself first, completely ignoring Statement 2, and decide only whether it is sufficient to answer the question stem, not what the actual answer is. Then evaluate Statement 2 by itself, ignoring what you learned from Statement 1. Only if neither statement alone is sufficient do you consider them together. For example, given the stem is x an even integer, and Statement 1 says x is divisible by 4, Statement 1 alone is sufficient because any multiple of 4 is even. If Statement 2 says x is divisible by 3, that alone is not sufficient, since 3 and 9 are both divisible by 3 but only one is even. Because Statement 1 alone works and Statement 2 alone does not, the answer is (A). Sufficiency means the statement pins down a single definite answer to the stem's question, whether that stem asks for a specific value or a yes-or-no determination; a statement that allows for two different answers is not sufficient, even if one of those answers seems more likely.
Value Questions versus Yes/No Questions
Data Sufficiency stems come in two forms, and each is judged for sufficiency slightly differently. A value question asks for a specific number, such as what is the value of x, and a statement is sufficient only if it narrows the possibilities down to exactly one value; if a statement is consistent with two or more different values of x, it is not sufficient, regardless of how the values compare to each other. A yes/no question asks whether some condition holds, such as is x positive, and here a statement is sufficient if it guarantees a definite yes for every allowed case, or if it guarantees a definite no for every allowed case; what a yes/no statement cannot do and still be sufficient is allow some cases where the answer is yes and others where it is no. This distinction trips up many test-takers, because a statement that consistently yields no is just as sufficient as one that consistently yields yes. For example, given the stem is x positive, Statement 1 says x^2 = 16. This is not sufficient, because x could be 4 or -4, giving inconsistent yes/no answers. But if the stem instead asks is x^2 positive, the same statement x^2 = 16 is sufficient, because x^2 = 16 directly and consistently answers yes regardless of whether x is 4 or -4. Recognizing which form a stem takes, and what counts as a definite answer for that form, is the first judgment call on every Data Sufficiency question.
The C-Trap
The C-trap is a classic Data Sufficiency pattern in which combining the two statements feels natural and sufficient, but in fact one statement alone was already sufficient on its own, making (C) the tempting but incorrect answer. Test-writers construct C-traps by presenting Statement 1 in a form that looks incomplete in isolation, then offering Statement 2 as an apparent missing piece, when Statement 1 was actually enough by itself, often for a subtler reason than it first appears. Consider the stem is n a multiple of 6, with Statement 1 stating n is a multiple of 12, and Statement 2 stating n is even. A test-taker in a hurry might see that Statement 1 alone establishes multiples of 12, all of which happen to be multiples of 6, and correctly mark it sufficient. But a rushed test-taker might instead pair Statement 1 with Statement 2 out of habit, assume both are needed, and select (C), even though Statement 2 alone (n is even) is not sufficient and was never required, since Statement 1 alone already answers the stem. The defense against the C-trap is procedural discipline: always fully evaluate Statement 1 in isolation and reach a firm conclusion about its sufficiency before you let your eye drift to Statement 2, and do the same for Statement 2 in isolation before considering them jointly. Skipping straight to combining the statements is the single most common route into a C-trap.
Hidden Insufficiency
Hidden insufficiency describes a statement that looks sufficient at a glance but actually permits more than one answer once you check less obvious cases, particularly involving negative numbers, zero, fractions, or non-integers. A stem asking for the value of x, paired with a statement that x^2 = 25, looks like it pins down x, but in fact x could be 5 or -5, so the statement is not sufficient unless the stem or another statement restricts x to positive values. Similarly, statements involving inequalities can hide insufficiency: knowing that x is greater than 2 does not by itself determine whether x^2 is greater than 4, greater than some other threshold, or an integer at all, since x could be 2.5, 3, or 1000. A frequent hidden-insufficiency scenario in word problems involves a statement that seems to fix a relationship between two variables but actually leaves a free parameter; for instance, knowing only that a rectangle's area is 24 does not determine its perimeter, because many length-width pairs multiply to 24, including 4 by 6 and 2 by 12, which produce different perimeters. The defense against hidden insufficiency is to actively search for a second case that satisfies the statement but produces a different answer to the stem, rather than stopping as soon as you find one case that seems to work; if you can construct even one alternative case with a different result, the statement is not sufficient, no matter how natural the first case felt.
Key terms
- Data Sufficiency
- — A Data Insights question type asking whether given statements provide enough information to answer a question stem, rather than asking for the answer itself.
- Statement
- — One of two numbered pieces of information provided in a Data Sufficiency question, evaluated for sufficiency both alone and in combination.
- Sufficient
- — A statement or combination of statements that pins down exactly one definite answer to the question stem.
- Answer choice (A)
- — Statement 1 alone is sufficient to answer the question, but Statement 2 alone is not.
- Answer choice (B)
- — Statement 2 alone is sufficient to answer the question, but Statement 1 alone is not.
- Answer choice (C)
- — The two statements together are sufficient, but neither statement alone is sufficient by itself.
- Answer choice (D)
- — Each statement is independently sufficient to answer the question on its own.
- Answer choice (E)
- — The two statements, even combined, are not sufficient to answer the question.
- Value question
- — A Data Sufficiency stem asking for a specific numeric value, requiring a statement to narrow the possibilities to exactly one number to be sufficient.
- Yes/No question
- — A Data Sufficiency stem asking whether a condition holds, requiring a statement to guarantee a consistent yes or a consistent no across all allowed cases.
- C-trap
- — A Data Sufficiency pattern where one statement is actually sufficient alone, but the question is designed to tempt a test-taker into unnecessarily combining both statements.
- Hidden insufficiency
- — A statement that appears to pin down one answer but actually permits multiple different answers once edge cases like negatives or fractions are checked.
Exam tips
- Always evaluate Statement 1 completely alone, then Statement 2 completely alone, before ever considering them together; this habit is the single best defense against the C-trap.
- For yes/no stems, a statement that produces a consistent no for every case is just as sufficient as one that produces a consistent yes.
- Before marking a statement sufficient, actively hunt for a second valid case, especially involving negative numbers, zero, or fractions, that would produce a different answer.
- Do not solve for the actual numeric answer to the stem; a Data Sufficiency question only asks whether the information is enough, not what the value is.
- If a word problem's statement seems to fix a relationship like area or a sum, check whether multiple different pairs of underlying values could satisfy it before calling it sufficient.