More from the NYT on graduation rates, and how widely/wildly they vary from school to school:
http://www.nytimes.com/2009/09/09/business/economy/09leonhardt.html
As a point of reference, Puget Sound's graduation rate.
I suspect that some of this has to do with the fact that our smaller endowment means fewer merit scholarships. In other words, fewer students come here on financial aid we have been awarded solely on the basis of their GPA or SAT scores--students who are much more likely to graduate, and graduate early. That doesn't mean that we can't do better in getting them through and out the door.