Experimental GMAT Questions
The makers of the GMAT include experimental questions on the test. These questions do not count towards your score. They are only there to help test writers decide upon future GMAT questions.
Computer-Adaptive GMAT Questions
Most standardized tests begin with easy questions and then get progressively harder. Computer-adaptive tests are different. The computer-adaptive GMAT always begins by giving you a question that is of medium difficulty. If you get it right, you'll get a harder question. If you get it wrong, you'll get an easier question.
Answering GMAT Questions
The computer-adaptive GMAT tallies your score as you take the test. The score calculation is based on the number of questions that you get right and their level of difficulty. If you leave GMAT questions blank, it will lower your score.