by mdinerstein Tue Oct 23, 2007 12:09 pm
While I am not an instructor, I do work at ManhattanGMAT as a Student Services Representative and I do know quite a bit about the difference between the actual GMAT exam and ManhattanGMAT CAT tests.
Unfortunately, there's no easy answer regarding the difference between the real GMAT and Manhattan GMAT's CAT exams. The standard error for the actual GMAT is roughly 30 points, meaning that you can score 30 points below or above whatever score you received and statistically be the same test-taker. Hence, if your "true" ability level is a 670, you can score anywhere in the range of 640 - 700 without ever improving your test-taking abilities. This is because the exam could slightly prefer one type of question over another. Therefore, if you are good at Geometry and receive one or two more Geometry questions than usual, you will perform better. The converse is true if you are bad at Geometry and see more of those types of questions.
The ManhattanGMAT Cat tests have a standard error of 50 points, so if your "true" ability level is at a 670, so you will be statistically the same test-taker if you score anywhere in between 620 and 720. This is due to both the question bias previously discussed and the fact that we aren't the GMAC and our questions will not be found in any official study material. Keep in mind, however, that we constantly revise our CAT pools and calibrate our exams to better reflect the current state of the GMAT, so we know that our practice exams are as accurate as possible. The reason you get varied student experiences with our tests is because some perfectly fall down a consistent testing line, leading to more positive feedback, and others fluctuate around the standard error, leading to more negative feedback.
Regardless of an individual's actual score on our exams, what we always emphasize is the fact that the assessment reports that draw from CAT exam data are vastly more useful in helping you improve because they break down your performance based on time spent on particular question. This way, you can see for yourself not only which subject areas need improvement, but which aspect of your test-taking abilities need work.
Best,
Michael Dinerstein