David Kuntz developed the algorithm for Knewton’s GMAT prep course. He is one of the brilliant brains behind the accuracy of Knewton CATs. This is the final installment in his CAT FAQ. For more info, check out parts 1 and 2.
How can my overall percentile be higher than both my quantitative and verbal percentiles?
Your overall score is calculated separately from your section scores, so you can score in the 99th percentile on the GMAT even if you didn’t score in the 99th percentile on either of the sections. For example, you could get a 48 on Quantitative (86th percentile), a 45 on Verbal (98th percentile), and a 760 overall (99th percentile).
Are the quantitative and verbal sections weighted equally in the total score?
Technically, yes — the estimates of your quantitative and verbal abilities that the CAT produces contribute the same amount to your overall score. However, the verbal section has a greater effect on your percentile rank because it is generally more difficult. If, for example, you scored a 40 on both the Quantitative and Verbal sections, your percentile rank for Quantitative would be 61st, but for Verbal it would be 91st. Your overall score (650) would be in the 84th percentile.
Why are scores above 51 rare? Why does the scale go up to 60? Can anyone get a 52?
For psychometric reasons, GMAC has truncated the scale at 51 (they do not report section scores higher than 51).
Why is it so difficult to create a good CAT?
A CAT needs to do many things well in order to reliably and accurately estimate your ability. It requires a robust algorithm to estimate your ability, a complex but speedy mechanism to identify the best question for you to see next, a rich pool of questions from which to select the questions, and a powerful scoring algorithm that translates the ability estimate into something meaningful.
Each test question has many characteristics that need to be simultaneously considered in the selection. The statistical characteristics of the questions all need to be determined beforehand through a process known as pretesting. Many, many questions are needed in order to be able to provide accurate assessment for all ability levels. And all of those questions need to be carefully constructed, reviewed, and statistically aligned so that they contribute meaningfully to your ability estimate.