CogAT Quantitative common mistakes

CogAT Quantitative · Understand the traps and how to avoid them.

Mistake 1

Assuming a variable is positive

A sign assumption can remove valid cases and make insufficient information look sufficient.

List the stated domain and test permitted positive, negative or zero cases where they can change the requested answer.

Mistake 2

An example can fit a proposed rule without showing that all allowed cases have the same outcome.

Seek a permitted counterexample or explain why the algebra covers every value in the stated domain.

Mistake 3

The difference between two rates and a relative change from a baseline use different denominators.

Write the requested comparison as a fraction or difference before computing, and label the resulting unit.

Mistake 4

Dividing by a variable expression silently assumes that expression is nonzero and can discard a valid case.

Factor or separate the zero case first, then check all candidates in the original relation.

Mistake 5

Distinct-looking algebraic forms or unreduced fractions can represent the same value or solution set.

Simplify and compare every plausible option, checking original restrictions before accepting a unique key.

Mistake 6

Groups with different counts contribute different numbers of observations to the combined total.

Recover each group sum from its count and mean, add the sums and divide by the combined count.

Mistake 7

A numerical association can arise through confounding, selection or chance without proving the proposed mechanism.

State the observed pattern and identify additional evidence or design needed for a causal conclusion.

Mistake 8

A familiar test format, historical belief or external example can contradict the actual supplied passage or model.

Use the question’s sources and restrictions first, and leave current exam policy and unsupported facts unspecified.