Your scaled score will change with each sitting, as it is based in part on how others do who take the same test. Therefore, even if you get exactly the same number of problems correct each time, your score can vary. It's also dependent on which problems you get correct, as they are not all equally weighted. As you may have noticed, the test gives you increasingly difficult problems depending on how many in succession you answer correctly. So, you know you are missing some correct answers if you suddenly find yourself with very easy questions. All in all the formula is very complicated, so you will go insane trying to crack the code when comparing other test scores with your own.

Thanks Bryant,this will help me.