Brent's post above is (as always) very good advice. To the reasons he outlines for taking a prep test early on, I'd add that you want to know the balance of your Quant and Verbal skills to apportion your study time most effectively. Verbal scores are much lower, on average, than Quant scores, so you should not just compare your raw scaled scores; instead look at the percentile charts at the back of any Official Guide.
After that initial test, there really is no good reason to continue taking practice tests until you've done a thorough content review. You should ask yourself *why* you're taking tests. Tests are not a good way to learn concepts. Especially early in your preparation, it makes much more sense to review a topic, then do questions on that topic, than it does to do questions which switch from topic to topic at random, as on a simulated test. The main reason to take practice tests is to practice your test-taking strategy, and especially your pacing strategy. If you struggle with pacing, you'll want to work on your pacing strategy a lot by doing several realistic practice tests, and GMATPrep has the most realistic question bank. The one other reason practice tests are useful is that they give you an estimate of your likely score on test day, so you'll know if it's a good idea to sign up for a real test. For that purpose, you need diagnostic tests which give accurate scores, and GMATPrep is the most accurate test available.
So, since you don't need to practice pacing early in your studies - after all, if you don't know the material well, of course you're going to struggle with timing on a lot of questions - taking repeated tests early on is not a good idea. It's after you've learned the material that you want to focus on test-taking practice. By that time, you likely will have forgotten the questions you saw on the GMATPrep you took at the start of your prep, so you haven't 'wasted' a test at all. Further, the question bank is large enough in each test that you can take each several times without seeing especially many repeated questions. Even the first question on the Quant test is selected from a batch of 4 questions, so in theory you could take each test 4 times and have a different first question each time.
You really want to (mostly) save GMATPrep for later in your study, because it's the only realistic practice test: it gives the most reliable score estimates, and the most realistic pacing practice. If you exhaust GMATPrep early in your prep, as one person suggested above, you won't have any good tests left when you really need them. The questions in GMATPrep are absolutely the most useful questions around - they're more recent, in general, than the OG questions, and there is a larger supply of high-level questions in GMATPrep than in the OG - but early in your preparation there are a ton of good questions to practice from (in the Official Guides, the old paper tests, from company providers, etc). You don't need to see GMATPrep questions early on. Later in your prep, though, you should get as much value from the GMATPrep questions as you can.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com
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