You are not getting actual ranking from the NRC rather two ranking ranges: Statistical-based (R-ranking for "Regression") and Reputation-based (S-ranking for "Survey"). For example, Northwestern CS has a 90% chance of being ranked between 42nd and 73rd (R-ranking) and between 26th and 69th (S-ranking).
Even with these wide ranges there are a number of problems in CS rankings: no citation information is being used, the data is five years old and much of the information is inaccurate (as the Pontiff complains). A simple sanity check on any ranking of CS departments would list the top four as some permutation of MIT, Berkeley, Stanford and Carnegie-Mellon. Congrats to Princeton for breaking this check.
Yesterday the CRA released a statement on the reliability of the CS rankings.
CRA is pleased that the NRC acknowledges there are errors in the data used to evaluate computer science departments and that, in the words of NRC Study Director Charlotte Kuh, “There’s lots more we need to look at for computer science before we really get it right.”Schools are taking the opportunity to promote their rankings. Northwestern Engineering boasts how well they did with some nice charts. We're certainly not the only ones.
Did the NRC fulfill their main mission of giving a valuable alternative to the US News rankings? US News uses a "wisdom of crowds" approach by just surveying department and graduate program chairs. The NRC tried a scientific approach which led to years of delays, many complaints about methodologies and accuracy, and a lack of a true ranking. After all the measures we can feed into a formula, the one thing that draws students and faculty to a department is its reputation. Complain as you will about US News, they do try to capture that one statistic.
Meanwhile what do I say to prospective graduate students who cite the low Northwestern CS numbers? I could mention the problems in the methodology, point to the CRA statement, say the numbers are based on data that predates most of the theory group. More likely I will fall back to that trite but true statement: You shouldn't choose a graduate program for their numbers, but for their people.