I'm trying to stay somewhat anonymous, but I checked our internal statistics and they don't match the ones posted by Chuck at all. (ETA: Not that those numbers weren't interesting and useful, Chuck!) I think it's because the size of our cohorts was increasing from 2002-2006, so if a program is growing these numbers won't match up well either. The information is still useful though, because if a program won't release attrition statistics, they might still be willing to tell you if the cohort size is increasing or if completion time has been increasing. Combined with the NRC information, you can make an educated guess about the program you're interested in.
Also, we have a former Northwestern student who transferred to our program who isn't reflected in the original statistics reported, but should be.
Take everything reported by a school with a grain of salt. My program loves to trumpet the time to matriculation, but the real story is much more complicated.