The GMAT computer adaptive test (CAT) is not just a computerized version of a paper-and-pencil test. On the GMAT, the test actually adapts to your performance as you are taking the test.
When you begin the GMAT Test, the computer assumes you that have an average score and gives you a question of medium difficulty. As you get answers correct, the computer serves up more difficult questions and estimates your ability. And vice versa, as you answer incorrectly, the computer serves up easier questions and decreases its estimate of your ability. Your score is determined by an algorithm that calculates your ability level based not just on what you got right or wrong; but also on the difficulty level of the questions you answered which is the best part about GMAT computer adaptive test.