Gre · Prep Guide
Read this first
Quantitative Comparison (QC) is roughly a third of GRE Quant, and it is the format that punishes fast, careless computation. This bible teaches the method — simplify, then test cases — and the named traps that make a wrong choice feel right. Work the practice set at the end as a mini-diagnostic.
The format on one page
| Choice | Meaning |
|---|---|
| A | Quantity A is greater |
| B | Quantity B is greater |
| C | The two quantities are equal |
| D | The relationship cannot be determined from the information given |
| The four choices are the SAME on every QC question — memorize them. | |
What Quantitative Comparison is
A Quantitative Comparison (QC) question shows you two quantities — Quantity A and Quantity B — sometimes with shared information above them, and asks a single question: which quantity is greater, or can you even tell? QC lives inside the Quantitative Reasoning measure (scored 130–170) and is one of four Quant question types. What makes it unique is the answer set: it never changes.
(A) Quantity A is greater
(B) Quantity B is greater
(C) The two quantities are equal
(D) The relationship cannot be determined from the information given
A.What is on the screen
- Optional shared information (an equation, a figure, a constraint) that governs both quantities.
- Quantity A and Quantity B side by side.
- The same four labelled choices (A / B / C / D) — you never read new options.
B.What makes QC different
- You are not asked to solve for a value — only to compare.
- Because the choices are fixed, every wrong answer is a reasoning error, not a mis-calculation.
- Choice (D) exists only here — a whole answer that means "it depends."
Why it rewards reasoning, not computation
Because you only need the relationship between A and B, you can almost always skip full computation. Cancel what both sides share, estimate, or test a value — then read off the label. Grinding out both quantities in full is the classic time sink QC is designed to punish.
Shared info: . Quantity A: Quantity B:
Subtract the common from both sides — a legal move that never changes which is larger. You are left comparing against . Quantity A is greater for every allowed , so the answer is (A). No value of was ever needed.
When (D) is right — and when it is a trap
Choice (D) is correct only when the relationship genuinely changes depending on the allowed values. The test rule is exact: if you can find one case where A is greater and another case where B is greater or equal, a single counter-case forces (D). But if the values are pinned down — or a figure looks like it settles things — (D) becomes a trap that punishes lazy or over-cautious guessing.
| Situation | Correct call | Why |
|---|---|---|
| An unconstrained variable could be negative, , or a fraction, and the comparison flips | (D) is right | You found two cases that disagree — the relationship truly is undetermined. |
| Shared info fully pins both quantities to numbers | (D) is a trap | One definite relationship exists; “it depends” is false. |
| A geometry figure looks like a right angle or a square, but nothing is stated | Usually (D) | Figures are not to scale — the drawing can hide a second case. |
| Only one test value was tried and it “worked” | Do not stop | “Consistent so far” is not “always”; seek a second, different case. |
- Read the shared info first. Every stated constraint (, “integer,” a marked angle) shrinks the cases you must check.
- Simplify by comparison. Cancel common terms; do not compute both sides in full.
- Stress-test with the value battery. , , a negative, a fraction — if the comparison flips, choose (D).
- Trust marks, not looks. On geometry, use only what is stated or marked; figures are not drawn to scale.
Compare, don't compute
| Choice | What it claims |
|---|---|
| (A) | Quantity A is greater |
| (B) | Quantity B is greater |
| (C) | The two quantities are equal |
| (D) | The relationship cannot be determined from the information given |
- Simplify both sides in parallel. Cancel or add the same amount to each quantity, reduce fractions, factor — anything that makes the two easier to line up. If both quantities share a term, drop it from both; the comparison is unchanged.
- Compare structure, not final values. Often you can see which side is larger without computing either — e.g. a positive times a bigger positive is larger. Stop the moment the direction is clear.
- If a variable is unconstrained, test cases. Run the sign/zero/fraction battery (next page). This is the single decisive habit in QC.
- If any two cases disagree, choose (D). One case where A wins and another where B wins (or they tie) is a proof that the relationship is not fixed — the answer is (D).
| Test value | Why it can break a comparison |
|---|---|
| A positive (e.g. 2) | The “obvious” case; growing an exponent grows the value. |
| Zero (0) | 0 is neither positive nor negative; it zeroes out products and powers. |
| A negative (e.g. −2) | Odd powers stay negative; even powers turn positive — signs flip. |
| A fraction (e.g. 1/2) | Between 0 and 1, higher powers shrink: x^3 < x^2 < x. |
Quantity A: Quantity B: (no restriction on )
Test : , → A is greater.
Test : , → B is greater.
Two allowed values give opposite outcomes, so the relationship is not fixed. Answer: (D). Choosing (A) here is the classic mistake — assuming an unnamed variable must be a whole number greater than 1.
Quantity A: Quantity B: given x < 0
Now the sign is fixed. For any negative : x^2 > 0 while x^3 < 0, so Quantity A is positive and Quantity B is negative.
Test : , . Test : , . Every case agrees. Answer: (A).
Same-looking problem as WEX 1, opposite verdict: the constraint x < 0 removes the cases that would have flipped it. Read the given information before you case-test.
Setup: a price of 100 is increased by 20%, then the result is decreased by 20%.
Quantity A: the final price Quantity B: 100
Compute on the new base each step: . So Quantity A , which is less than 100.
Answer: (B). The trap is (C): a then feels like it cancels, but the second percent is taken on the larger base (120), so it removes more than the first added. Percentages do not cancel unless they act on the same base.
Every wrong choice is a named trap
In a Quantitative Comparison item the four answer choices never change: (A) Quantity A is greater, (B) Quantity B is greater, (C) the two quantities are equal, and (D) the relationship cannot be determined from the information given. Because the labels are fixed, ETS cannot tempt you with a mis-typed number — it tempts you with a flawed line of reasoning. Learn the flaw and the choice loses its pull.
Working the sign battery
Suppose the only information is . Compare Quantity A: with Quantity B: .
- The tempting read: ", so is small, and squaring a small number makes it smaller — pick B." That silently assumes is a positive fraction.
- Run the battery. Try x = 1/2: then x2 = 1/4 < 1/2, so B is greater.
- Now try (still satisfies ): then , so A is greater. The comparison flipped.
- One case gave B, another gave A — the relationship is not fixed. The answer is D.
The distractor B is not a wrong subtraction; it is the unconstrained-sign assumption dressed up as arithmetic.
The geometry and one-case traps
The arithmetic and statistics traps
A price of rises 20%, then the new price falls 20%. Compare Quantity A: the final price with Quantity B: .
| Step | Base used | Computation | Result |
|---|---|---|---|
| Start | — | — | |
| Up 20% | |||
| Down 20% |
The final price is , so Quantity B is greater — the answer is B, not C. The down-step was 20% of (), a bigger cut than the the up-step added. Symbolically the net factor is , a 4% loss regardless of the starting number.
Counting, and the full trap map
| Trap | The false assumption it rewards | Disconfirming check |
|---|---|---|
| Unconstrained-sign | A variable is positive / an integer; squaring always grows it. | Run the 0 / 1 / negative / fraction battery; if it flips → D. |
| Figure-not-to-scale | The drawing preserves angles and proportions. | Use only marked facts; redraw to an extreme. |
| Insufficient-cases | One test value settles the comparison. | Find a second, dissimilar case (flip a sign, cross zero). |
| Percent-base-shift | +20% then −20% returns to the start. | Recompute on the new base: . |
| SD-vs-range | A larger range means a larger standard deviation. | SD measures spread about the mean, not the endpoint gap. |
| Perm-vs-comb | Selections are counted as ordered arrangements. | Does reordering make a new outcome? No → combination. |
GRE glossary
| Term | What it means |
|---|---|
| Quantitative Comparison QC | A GRE Quant question type that asks you to compare Quantity A and Quantity B and choose which is greater, whether they are equal, or whether the relationship cannot be determined. |
| Quantity | One of the two expressions being compared in a QC question, labeled Quantity A and Quantity B; each may be a number, an expression, or a value defined by a figure or condition. |
| Cannot be determined | Answer choice (D): the relationship between the two quantities changes depending on allowed values, so no single comparison holds. Valid only when at least two permitted cases give different comparisons. |
| Fixed answer choices | The four QC options — Quantity A greater, Quantity B greater, the two equal, or cannot be determined — which are identical and in the same order on every QC question. |
| Not necessarily to scale | The convention that geometric figures (lines, circles, triangles, quadrilaterals) may not reflect true lengths or angles; base answers on geometric reasoning, not on appearance. |
| Drawn to scale | The convention that coordinate systems (xy-planes, number lines) and statistical graphs (bar, circle, line graphs) do reflect true values, so you may read or estimate quantities by sight. |
| Term | What it means |
|---|---|
| Unconstrained variable | A variable with no stated restriction on its sign, integrality, or size; it may be negative, zero, or a fraction, which is the root of most QC traps. |
| Test cases | The technique of substituting specific allowed values — chosen to probe sign, zero, and fractions — to see whether the comparison stays fixed or changes. |
| On-screen calculator | The basic four-function-with-square-root calculator provided for the entire Quantitative Reasoning measure; it is not a graphing calculator and is best reserved for tedious computation. |
| Section-level adaptive | The GRE design in which the difficulty of a measure's second Quant section is selected from your performance on the first; adaptation is per section, not per question. |
| Scaled score | The reported Quantitative Reasoning score from 130 to 170 in 1-point increments, reflecting both correct answers and the difficulty of the sections received. |
| Geometric reasoning | Deriving lengths, angles, and areas from stated facts and theorems (e.g., the Pythagorean theorem) rather than from measuring or eyeballing a not-to-scale figure. |
Frequently asked questions
What is a GRE Quantitative Comparison question?
A Quantitative Comparison (QC) question presents two quantities — Quantity A and Quantity B — and asks you to determine how they compare. Instead of computing an exact value, your job is to decide which quantity is greater, whether they are equal, or whether the relationship cannot be determined from the information given.
How many answer choices does a Quantitative Comparison question have?
Exactly four, and they are always the same, in the same order: (A) Quantity A is greater; (B) Quantity B is greater; (C) The two quantities are equal; (D) The relationship cannot be determined from the information given. Unlike other GRE Quant questions, QC never has five choices and never varies its options.
Are the figures in GRE Quantitative Comparison drawn to scale?
It depends on the figure. Geometric figures — bare lines, circles, triangles, and quadrilaterals — are NOT necessarily drawn to scale, so you must reason from the given facts, not from how they look. Coordinate systems (xy-planes and number lines) and statistical graphs (bar, circle, and line graphs) ARE drawn to scale, so you may read, estimate, or compare values by sight in those.
Can you use a calculator on GRE Quantitative Comparison?
Yes. A basic on-screen calculator (add, subtract, multiply, divide, square root) is provided for the entire Quantitative Reasoning measure, which includes QC. It is not a graphing calculator. ETS advises using it only for genuinely tedious computation, because most QC questions are decided by reasoning, not arithmetic.
How is GRE Quant scored?
The Quantitative Reasoning measure is scored on a scale of 130 to 170 in 1-point increments. Your score reflects both how many questions you answer correctly and the difficulty of the sections you receive, because Quant is section-level adaptive. There is no separate score for Quantitative Comparison on its own.
What is the single biggest mistake on Quantitative Comparison?
Assuming a variable is positive, an integer, or non-zero when nothing in the problem says so. An unconstrained variable can be negative, zero, or a fraction, and each of those can flip the comparison. The fix is to test cases across sign, zero, and fractions before you commit to an answer.
When is the answer 'cannot be determined'?
Choose (D) only when you can produce two allowed cases that give different comparisons — for example one case where Quantity A is greater and another where the quantities are equal. If every case you can construct yields the same relationship, the answer is not (D). Note that if both quantities are specific numbers with no variables, (D) can never be correct, because two fixed numbers always have a definite relationship.
Should I guess on Quantitative Comparison?
Yes — always answer. GRE Quant has no penalty for a wrong answer, so a guess can only help, and QC's four fixed choices make an educated guess especially efficient after you eliminate one or two options with a single test case.
Where to go from here
You now understand Quantitative Comparison better than most GRE test-takers ever will — the four fixed choices, the case-testing method, and the traps that make a wrong choice feel right. The points come from reps.
| Do this next | Why |
|---|---|
| Take an official ETS POWERPREP practice test | Convert the QC method into reflexes under the real timer. |
| Drill Text Completion & Sentence Equivalence | The Verbal formats where vocabulary and traps decide the score. |
| Re-read the not-to-scale rule | Never solve a QC geometry question by measuring the picture. |
| Drill traps in the AskSia app | Per-distractor coaching on why you miss — bilingual, the part a guide can’t give. |