Are the GMAT Club Tests
listed in order of difficulty (1 the easiest to 25 the hardest)?
No, they are not in the increasing order of difficulty. Each test is designed to be fairly balanced but we know there are harder tests than easier ones. You can use percentiles to give you an idea where you stand but don't get too discouraged with your correct/incorrect ratio - the tests were designed to be hard (very tricky).
Founder of GMAT Club
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Co-author of the GMAT Club tests
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