I don't think that dental ranking is impossible, but that it's more trouble than it's worth to companies who would market a book for sale to pre-dents. There are many, many more medical schools and medical school applicants out there than dental school. The target audience is smaller for dental school, and the profit margin/expenditure in research would be lower. Also, the nature of dental school would require them to send out people to qualitatively determine clinic quality, clinical skills requirements, and etc, b/c this is one of the largest determinants for many as to whether or not a school is ranked high on their wish list.
Some schools would definitely express reticence in releasing such information, because it would not be to their advantage. The qualitative measures of assessment would be GPA, DAT, average NDBE I and II, class pass rates, % of class that graduates on time, externship opportunities, specialization rates, and etc. Some schools as of now aren't even willing to share on these. And that which they do allow access to, is already readily available on their website. It's not that it's impossible to rank dental schools, but dental schools may not want to release such information in an already smaller pool of available graduate programs.
