We work really hard to make our practice exams as close to the actual GMAT as possible. Our curriculum development team make sure to keep on top of any new changes in the test and we do mimic the algorithm of the test to the best of our ability. As a result, our test has a standard deviation of around 50 points.
From feedback we've gotten from students as well as on places like this and other forums, we do feel that our tests are quite accurate and are a good measure of your current ability level.
Later in your studying, I would definitely suggest taking the GMAT Prep tests (available on mba.com) as they are put out by the GMAC (who make the test) so they are the most accurate practice tests you can take. But we usually suggest to our students that they save those two tests until the end of their studying so that they have the best perception of what they can expect going into the GMAT.
In the meantime, make sure to study the material, so that you can see an improvement for the next practice test you take.
You may find this article, on planning and executing your GMAT preparation, useful: http://www.manhattangmat.com/strategy-s ... t-prep.cfm
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Caitlin Clay | Manhattan GMAT Student Services Associate | New York
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