In analyzing federal data for an in-depth examination of the nation's historically black colleges and universities, The Atlanta Journal-Constitution found that the six-year graduation rates at 21 schools were 20 percent or lower in 2015.

This means that four of five beginning freshmen at those schools didn’t earn a degree within six years.

Coming Thursday: The AJC publishes HBCUs: A Threatened Heritage. The special three-part series will examine the problems many of the schools face, the unique role they play in students' lives, and the perils that lie ahead. 

handful of HBCUs are graduating more than 50 percent of new freshmen within six years. Spelman College in Atlanta, for example, led all HBCUs, with a rate of 76 percent. Others, however, are struggling. Here's a sampling:

“Yes, there are some HBCUs that have low graduation rates,” Marybeth Gasman, an education professor at the University of Pennsylvania who directs the Penn Center for Minority Serving Institutions, said in an email to the AJC. “And some that are in the single digits. . . . This is problematic and a school must do better by having summer bridge programs, peer-to-peer mentoring, student success centers — all focused on increasing retention and graduation rates.”

Gasman pointed out that some schools with low graduation rates in 2015 already have begun to turn things around.

She also wrote of the connection between graduation rates and family income:

“Graduation rates directly correlate with the income of the student body. More low income students — typically, lower graduation rates. Why? Because low-income students don’t have access to the same college prep opportunities and because they don’t have the financial safety nets of middle and upper income students. Please note that institutions that have very few Pell Grant-eligible students typically have very, very high graduation rates.”

Pell Grants are awarded to low-income undergraduate students.