I know I've mentioned this a lot on the forums, but it's really important to understand: the official test has a standard deviation of 30 points. That is a wide standard deviaton. This test in general is not an "exact score" kind of test.
A 650 on a test with a 30-point standard deviation means that, if you were to take it again, you'd have a 2/3 chance of scoring in the 620 to 680 range and a 1/3 change of scoring below 620 or above 680. Again, that's a wide range.
Our test has a standard deviation of about 50 points. The GMATPrep standard deviation has not been published, but it is going to be higher than the SD of the official test (30). I always assume GMATPrep is also going to have a better standard deviation than ours, so I think of it as 40 points (though, again, this is just a guess - the figure hasn't been published for GMATPrep).
So Enginpasa1, you are scoring in the range 560 to 660 right now according to your MGMAT results. DrivinWest, the fact that your GMATPrep was only 10 points off from your real GMAT is good for you but not the typical case - the range is a lot wider than that. If my guess about GMATPrep's SD is right, that 660 means that you were scoring in the 620 to 700 range when you took that practice test.
It really doesn't get more specific than that - some people get scores very close to what they scored on GMATPrep or what they scored on our tests, and some people don't. I have students who score at or above what they were scoring on our tests all the time (and others who score below, of course). It really varies - that's why we publish our standard deviation and try to educate people about the SD of the official test.
Also, things WILL be different on test day b/c you know it really counts this time. That also affects different people differently.
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Stacey Koprince
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Manhattan GMAT
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