📊 Data Insights

Data Sufficiency

Classify whether the data answers the question without solving it: the five options, value vs yes/no sufficiency, and statement quarantine.

~6h

to master

The format that punishes solving

Data Sufficiency (DS) gives you a question plus two statements, (1) and (2), and asks a single thing: is there enough information to answer? You never compute the final answer — you classify how much the statements provide. The five options are identical on every DS question:

📐Core Rule

The five answers — memorize them as positions, not sentences. (A) (1) alone is sufficient, (2) alone is not. (B) (2) alone is sufficient, (1) alone is not. (C) Together sufficient, neither alone. (D) Each alone is sufficient. (E) Even together, not sufficient.

A clean mental flowchart: check (1) alone → check (2) alone → if exactly one works, it's A or B; if both work, D; if neither alone works, test them together → C if together suffices, else E.

What "sufficient" means depends on the question

📐Core Rule

Value questions ("what is $x$ ?") need the data to pin down exactly one number. A statement that leaves two possible values is not sufficient. Yes/No questions ("is $n$ even?") need a definite always-yes or always-no. "Always no" is just as sufficient as "always yes" — uncertainty is the only failure.

This single idea decides most DS questions. For a value question, find two allowed values and the statement dies. For a yes/no question, find one case giving "yes" and one giving "no" and it dies.

💡Exam Tip

Quarantine the statements. Evaluate (1) with (2) completely erased from your mind, then (2) with (1) erased. Letting information leak between them is the most common DS error — and it's exactly what separates a true (C) from a disguised (A), (B), or (D).

Try the discipline on and on .

The techniques that crack DS fast

Test numbers. For value questions, hunt two cases that satisfy the statement but give different answers → not sufficient. For yes/no, hunt a yes-case and a no-case. Choose deliberately different numbers: include 0, negatives, fractions, and equal values.
Don't finish the arithmetic. Once you know a unique answer exists, stop — computing it wastes 30 seconds you don't have. DS rewards the judgment, not the number.
Rephrase the question first. "Is $\frac{x}{y} > 1$ ?" with unknown signs is really "is $x > y$ and is $y > 0$ ?" Simplifying the target before reading the statements saves whole questions.

⚠️GMAT Trap

(C) is a trap when one statement already suffices. Habitually reaching for "both together" feels safe but is wrong whenever (1) or (2) works alone — that's the difference between (C) and (A)/(B)/(D). Always ask "do I actually need the second statement?"

⚠️GMAT Trap

Hidden constraints decide hard DS. " $x$ is an integer," "the figure is a rectangle (not necessarily a square)," "the quantity is positive." Sufficiency often turns entirely on whether such a constraint is present. And never assume a figure is drawn to scale — a shape that looks square may not be.

✏️Worked Example

Why "no" is sufficient. Question: Is the integer $k$ odd? Statement: $k$ is the product of two consecutive integers. A product of consecutive integers is always even, so the answer is a definite No — the statement is sufficient, even though it never lets $k$ be odd. Sufficiency is about certainty, not about getting "yes."