GRE quantitative comparison: four traps you can spot in 20 seconds

You’re scoring around 155 in Quant, your practice-test reviews keep showing the same thing, and the misses are mostly quantitative comparison. The four traps below account for most of those points, and each one has a check you can run in about 20 seconds before you commit an answer.

Quantitative comparison is the format that makes the GRE’s Quant section different from every math test you took in school. ETS describes it on its Quantitative Reasoning page: you’re shown Quantity A and Quantity B, sometimes with shared information centered above them, and you choose whether A is greater, B is greater, the two are equal, or the relationship can’t be determined. Those four choices never change. That fixed structure is what makes the traps so repeatable, and so learnable.

Why QC is where a 155 leaks points

At 155 you can do the math. What you can’t yet do is stop doing the math at the right moment, and QC punishes that in both directions. Calculate too much and you run out of time for the data interpretation set at the end of the section. Calculate too little and you pick an answer based on how the quantities look rather than what they must be. Neither error is about content.

The questions also look easier than they are. A QC item with two short expressions reads as a 30-second question, and the test writers know that, so the shortest-looking items often carry the trap. The habit that fixes this is simple to state: before answering any QC question, ask what would have to be true for the answer to be D, and then check whether it is. Four traps do most of the damage.

Trap one: plugging in only the friendly numbers

When a quantity contains a variable, most people test it with 2 or 3, get an answer, and move on. The GRE is built to punish exactly that, because the relationship between two expressions often changes at zero, at one, between zero and one, or below zero.

The 20-second check is a fixed list of test values you run every time a variable appears: 0, 1, a fraction between 0 and 1, and a negative number, subject to whatever the shared information allows. In the example above, the shared information rules out 0 and negatives, which leaves 1 and 1/2, and either one breaks the answer A.

This trap also hides inside exponents. Anything raised to an even power behaves differently for negatives than the base does; anything between 0 and 1 gets smaller when you square it. If a quantity has a variable under a power, the fraction case and the negative case are the two you can’t skip.

Trap two: the answer-D trap

Answer D exists for questions where the relationship genuinely depends on an unknown. People in the 150s reach for it in two wrong situations: when the problem feels uncertain because the numbers are ugly, and when a variable is present but the relationship is actually fixed.

The first case has a rule you can apply without thinking. If both quantities consist entirely of numbers, with no variables and no unknown figures, D is impossible. Each quantity is a specific value, so one of A, B or C must be right, and the only question is which.

The second case needs a little algebra rather than a rule, and it catches more people than the first because the presence of a variable makes D feel legitimate. Simplify before you judge.

The check is the same question from a different angle: can I find two allowed values that produce different relationships? If both quantities are pure numbers, you can’t, so D is out. If you’ve simplified until the variable disappears or appears identically on both sides, you can’t, so D is out. D is right only when you’ve actually found the two cases, not when you’ve merely failed to find an answer.

Trap three: figures that are not drawn to scale

ETS states in its Quant instructions that geometric figures are not necessarily drawn to scale, and that you should not assume lengths or angles from how a figure looks. People read that once and then spend the test trusting their eyes anyway, because a drawing is persuasive in a way that a disclaimer is not.

What you may trust in a figure: the labeled values, the order of points on a line, and the fact that lines drawn as straight are straight. What you may not trust: that a segment that looks like a midpoint is a midpoint, that an angle that looks right is 90 degrees, that a triangle that looks isosceles is isosceles.

The 20-second check: cover the figure with your hand, or at least look away from it, and rewrite what you actually know as a list of equations and givens. Then ask whether the quantity you’re comparing can be derived from that list alone. If it can’t, the figure is doing the arguing, and the answer is probably D.

Trap four: ignoring the shared information

Many QC questions place a line of information above the two quantities. That information applies to both quantities, and under time pressure people read the quantities first, form an opinion, and never go back to the line above them. The result is usually a D that should have been A, B or C.

The check here is mechanical. Before you compare anything, read the centered line and restate it in your own words, including the quiet constraints: “positive” rules out zero and negatives, “integer” rules out fractions, “distinct” rules out equal values, “x > y” is an ordering you must keep. Then carry that restatement into your plug-in list from trap one. Most false D answers come from a constraint the test taker read but didn’t use. Five seconds of restating saves the question.

Putting the four checks together

The four checks compress into one sequence you can run on every QC item in about 20 seconds, before any real computation. Memorize the order.

1. Read the shared information and restate every constraint, including the implied ones.
2. Decide whether D is even possible: no variables anywhere means no.
3. If there’s a figure, list what’s given and stop looking at the picture.
4. If there’s a variable, simplify both quantities the same way until one side is obvious or the variable cancels.
5. If the variable survives, run the plug-in list: 0, 1, a fraction, a negative, within the constraints.
6. Answer, then move on without recomputing to “make sure.”

Step six is the one adults skip. Having found a relationship that holds for every case you tested, you don’t improve your odds by testing a fifth case; you spend time you’ll need at the end of the section. The discipline of QC is knowing when the comparison is settled.

Rick’s note: When I review a practice test with a student stuck in the mid-150s, I ask them to say out loud, for each QC miss, which of the four traps it was. Nearly every miss has a name. Once a student can name the trap while reading the question, the misses stop, and that usually happens within two weeks of practice, not two months.

How to drill this in two weeks

Pull every QC question from the official ETS practice material you have and do them in sets of ten, untimed at first, writing the trap name next to each item before you answer. For the first week, accuracy is the only goal. In the second week, time the sets at one minute per question and keep writing the trap name; the point is to make the diagnosis automatic at speed, so that on test day the 20-second check happens without your deciding to run it.

Log every miss by trap, not by topic. If the plug-in trap is costing you most of the points, your problem isn’t algebra, it’s the habit of stopping at 2 and 3, and the fix is to write “0, 1, fraction, negative” at the top of your scratch paper (or whiteboard, if you test at home) before the Quant section starts.

What to do this week

1. Write the four answer choices from memory and check them against the ETS Quant instructions, so you never read them during the test again.
2. Do ten official QC questions untimed, naming the trap before answering each one, and note how many you’d have missed without the check.
3. Go back through your last practice test and label every QC miss with one of the four trap names.
4. Put the plug-in list (0, 1, a fraction, a negative) on an index card and tape it where you study until you stop needing it.
5. Schedule two 30-minute QC-only sessions this week, one on accuracy and one at one minute per question.