One of the easiest ways to miss GRE Quantitative Comparison questions is to assume something the question never told you.

This happens constantly.

A student sees a variable and subconsciously assumes it is positive. Or assumes it is an integer. Or assumes it cannot be zero. Or assumes two quantities are ordered in the most “natural” way. Then the student compares Quantity A and Quantity B, chooses an answer, and misses the question — not because the math was hard, but because the setup was built on an assumption that was never guaranteed.

That is one of the biggest traps in Quantitative Comparison.

QC questions punish hidden assumptions aggressively.

For example, suppose a question gives you:

x < y

A lot of students immediately imagine two positive numbers, such as x = 2 and y = 5. But the question did not say x and y are positive. They could both be negative. One could be negative and one could be positive. One could be zero. Depending on the quantities being compared, those different cases may completely change the answer.

The same issue comes up with expressions such as x2, 1/x, |x|, or x + y. If you quietly assume x is positive, you may miss the entire point of the question.

This is why Quantitative Comparison is not just regular Quant in a different format. It is a test of mathematical caution. The GRE is often asking: “Do you know what must be true, or are you assuming what seems likely?”

That distinction matters.

If a variable is not stated to be an integer, do not assume it is an integer. If a variable is not stated to be positive, do not assume it is positive. If a variable is not stated to be nonzero, consider whether zero is allowed. If two numbers are not clearly ordered, do not invent an order. If a figure is not drawn to scale, do not trust how it looks.

And if a quantity “feels bigger,” ask whether it must be bigger in every allowed case.

That last phrase is key: in every allowed case.

In Quantitative Comparison, you are not trying to find one example that supports your preferred answer. You are trying to determine whether the relationship between the two quantities is always the same.

If Quantity A is greater in one case, but Quantity B is greater in another case, the answer is D: the relationship cannot be determined.

That is where many students get trapped. They test one convenient case, get a result, and stop. But one case is rarely enough unless the question structure guarantees that the relationship cannot change.

So, before solving a QC question, pause and ask:

• What do I actually know?
• What values are allowed?
• Are negatives possible?
• Are fractions possible?
• Is zero possible?
• Are the variables integers, or am I assuming that?
• Could different valid cases produce different relationships?

That short pause can prevent a lot of mistakes.

The GRE loves answer choices that reflect the most common assumptions students make. If most students assume a number is positive, there may be a trap answer waiting for them. If most students assume a diagram is drawn accurately, there may be a trap. If most students assume a variable is an integer, there may be a fractional case that breaks the comparison.

So, when you review missed QC questions, do not just ask, “Did I know the math?”

Ask, “What did I assume?”

That question is often more revealing.

You may find that your misses are not coming from weak algebra or weak arithmetic. They may be coming from hidden assumptions:

• “I assumed x was positive.”
• “I forgot x could be zero.”
• “I tested only integers.”
• “I trusted the diagram.”
• “I assumed the relationship from one example had to hold generally.”

Those are fixable habits. But you have to notice them first.

Quantitative Comparison rewards students who are precise about what is known and what is not known. It rewards students who test boundary cases, challenge their first instinct, and understand that “probably” is not good enough.

The standard is not: “Does this seem true?”

The standard is: “Must this be true?”

That is the mindset shift.

Very often, the key to a QC question is not doing more math. It is refusing to assume more than the problem gave you.