Data Sufficiency Reasoning: Deciding Whether Information Is Enough
What you must know
Data sufficiency asks whether the given statements provide enough information to answer the question definitively - not to compute the value. The five standard answer choices are: (A) statement 1 alone suffices, (B) statement 2 alone suffices, (C) both together suffice but neither alone does, (D) each alone suffices, (E) together still insufficient. You must evaluate each statement independently first, then combine only if needed.
Traps
- Solving instead of deciding: you don't need the value, only certainty that exactly one answer exists.
- Carrying information from statement 1 into statement 2 - evaluate each alone.
- Forgetting to test multiple cases: if two different scenarios satisfy a statement but give different answers, the statement is insufficient.
- Assuming integers or positives unless stated; non-integer or negative values often break a tempting conclusion.
- Confusing "sufficient" with "true" - a statement that yields a definite "no" is still sufficient.
Worked example
Question: Is x > 0? Statement 1: x² = 9. Then x = 3 or x = −3, giving different answers - insufficient. Statement 2: x³ = 27. Cubing is one-to-one, so x = 3, and the answer is definitively yes - sufficient. Answer: (B).
Rule of thumb
For each statement, deliberately hunt for two cases that produce different answers. If you find them, the statement is insufficient; if every case yields the same answer, it is sufficient.