Because DS is a question type unique to the GMAT, most students will initially perform worse on those questions at first, until they learn proper DS technique. You have to learn how to:
a) frame / rephrase the question correctly
b) analyze the statements with the correct mindset: trying to prove insufficiency
Here are the biggest problems I see students having with DS questions:
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Mixing up what they know with what they're wondering (i.e. assuming that the question is true and/or testing the validity of the statements). You always have to take the statements as irrefutable facts, and see what they tell you about the question.
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Not considering multiple cases. If you're testing numbers, be sure to consider negatives, fractions, square roots, and zero in addition to the postitive integers that tend to spring to mind first.
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The "Value Fallacy" - thinking you always need to solve for the value of the variable to answer the question. Most of the time with yes/no questions, you won't need a definitive value to get a sufficient answer of "yes" (or much less commonly, "no"). Especially if the question asks for a proportion (percent, ratio, or fraction), it's unlikely that you'll need a value to answer it.
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Falling for the "C trap." This is what we call it when students combine the statements too quickly without considering them individually. If it seems too obvious that the statements together are sufficient, then one of the statements is probably sufficient on its own.
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Statement carryover - accidentally using what you learned from statement 1 when you're evaluating statement 2.
Here are a few articles to help you think through these questions in the right way:
https://www.manhattanprep.com/gmat/blog/ ... ncy-works/
https://www.manhattanprep.com/gmat/blog ... questions/
https://www.manhattanprep.com/gmat/blog ... ncy-traps/
https://www.manhattanprep.com/gmat/blog ... est-cases/
https://www.manhattanprep.com/gmat/blog ... fficiency/
https://www.manhattanprep.com/gmat/blog ... aw-it-out/
https://www.manhattanprep.com/gmat/blog ... tatements/
https://www.manhattanprep.com/gmat/blog ... ust-right/
https://www.manhattanprep.com/gmat/blog ... at-part-1/
https://www.manhattanprep.com/gmat/blog ... ms-part-1/
https://www.manhattanprep.com/gmat/blog ... -part-1-2/
https://www.manhattanprep.com/gmat/blog ... fficiency/