I agree that benchmarks are important, especially when comparing one school with another (if a school enrolls a more challenging demographic, it can hardly be blamed for its lower rate). And you provide some reasons that a perfectly good school might have a low graduation rate.

But I am hesitant to embrace “rigor” as one of them. If a school is rigorous in its grading, but not similarly rigorous (or at least transparent) in its admissions — that is, it lets in students who are likely to fail, yet enroll thinking they’ll graduate — it’s not doing anyone any favors. There may be some highly specialized, highly demanding programs where a low graduation rate is acceptable on “rigor” grounds (students might think a 45 percent shot at earning a very valuable degree or certification is a good gamble), but that’s rare.