NCEs are just a different measure. Percentiles are really bunched up around the middle of the curve, whereas NCEs are equal-interval measures (based, as they are, on the standard deviation), so they're more spread out than percentiles are in the middle, but closer together than they are at the extremes. That's why out at the tails, a percentile of 98 has a lower NCE. Everything is 99 for DS because NCEs of 99 are defined to be at the 99th %ile. Percentiles are easier for most non-mathy people to understand, as they represent your place in line. The NCE is exactly as meaningful as any other transformation of the standard deviation/z score, such as the standard score used for IQs and achievement tests, or scaled score used for IQ subtest scores. It tells you your distance from the center of the bell curve (defined as NCE = 50). A NCE is like a standard score where the mean = 50, and the standard deviation = about 21.
It's value is that, as an equal-interval scale, it is perceived as being better for making comparisons between different measures, and for measuring growth.
