This is the last installment of an FAQ series on Computer-Adaptive Tests (CAT). Check out part 1 and part 2 as well.

How can my overall percentile be higher than both my quantitative and verbal percentiles? Why isn’t it the average of the two?

You can be in the 99th percentile overall even if you didn’t score in the 99th percentile on either of the sections, because the overall score is calculated separately from the section scores and their percentiles. So, 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, in that the estimates of your quantitative and verbal abilities that the CAT produces contribute the same amount to your overall score. However, the verbal section is generally more difficult, and therefore has a greater effect on your percentile rank. If, for example, you scored a 40 on both the Quantitative and Verbal sections, your percentile rank for Quantitative would be 61st, for Verbal would be 91st, and overall (650) would be 84th.

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.