The group Education Sector has a new report about graduation rates, and minority graduation rates in particular. There are a number of important lessons to draw from it, one being that fewer than two-thirds of college students graduate within six years. That is a lot of wasted time.

Also, for minorities, the number falls below the one-half mark. The report looks at a number of colleges that have various black-white graduation gaps, and tries to ascertain the differences between them. Not surprisingly, schools that provide intensive tutoring to low-income and minority students have smaller gaps.

However, the report leaves out some major factors. One is affirmative action — the words “affirmative” and “preference” appear nowhere in the document. Yet the more aggressively an institution overlooks academic qualifications in the name of diversity, the more it will increase its graduation gap.

Another is variations in the income gap. If college A has black and white students who come from similarly affluent families, it will have a more narrow graduation gap than college B, which tends to bring in poor black students and middle-class white students.

A third is sports. As Inside Higher Ed once documented:

Data drawn from the National Collegiate Athletic Association’s annual survey of graduation rates … show that scholarship athletes make up at least 20 percent of the full-time black male undergraduates at 96 of the nearly 330 colleges that play sports in Division I, the NCAA’s top competitive level. At 46 of those colleges, according to the data, which are from 2005-6, at least a third of the black male population play a sport. And at 31 one of them, football players alone make up at least a quarter of the black undergraduate men.

Clearly, at some schools, black athletes make up a higher proportion of the black population than at others — as athletes often don’t graduate, this can contribute to the overall race gap.