Americans who attended college for a while but never earned a credential might be the key to achieving the ambitious college completion goals the White House and influential foundations have set.

It’s a big group. More than 31 million people enrolled in college during the last two decades but left without earning a degree or certificate and have not returned to higher education for at least 18 months, according to new data from the National Student Clearinghouse Research Center.

Many dropped out quickly. Roughly one-third -- or 10 million -- of the identified noncompleters left college after enrolling for just a single term, according to the study, which the center released on Tuesday.

Among the remaining 21 million former students who attended college for more than a term, about 17.5 million failed to get beyond two years of academic progress. The remaining 4 million moved past the two-year mark.

This group, which the report calls “potential completers,” should be the most relevant to policy discussions around the national college completion push, the center said.

Typical Potential Completer 24-29 years old Left college 2-6 years ago Attended college for at least 2 years "Stopped out" at least once before leaving for good As likely to have attended 4-year college exclusively as 2-year college

For example, the report cites Project Win-Win. The Institute for Higher Education Policy conducts the project, which has tracked down 6,700 former students who had either earned enough credits for a degree or were within striking distance. As of last fall 4,500 students had received degrees through the program.

The most common type of potential completer is 24 to 29 years old and has been out of higher education for two to six years, the report found. About 600,000 women and 630,000 men fit this description. (Note: this paragraph has been changed from an earlier version to reflect corrected data from the center.)

More than one in four potential completers enrolled in college continuously or intermittently for seven years or longer. And the study found that about 36 percent spread their enrollments over four to six years.

“These results suggest that standard cutoffs for measuring student graduation rates (typically 150 percent of program length) are inadequate,” said the report. “Significant numbers of students continue to make substantial progress toward a credential for many years longer.”

More Data, More Questions

The new report advances what is known about the some-college, no-degree population. Until now the U.S. Census has been the primary data source on the group. The Census found that roughly one-fifth of working-age Americans have attended college at some point but do not hold a degree.

The Lumina Foundation has cited that number in its call for increased degree completion. But the center’s report, which Lumina funded, goes beyond that raw figure by showing more detail about former students’ experience in higher education.

The clearinghouse has access to student records for 96 percent of the total U.S. student population. More than 3,600 institutions provide information on enrollments and degree production to the nonprofit group, which conducts transcripting and research services for its member colleges.

As a result, the center was able to use the clearinghouse database to track virtually all students who had at least one enrollment record at a U.S. college during the last 20 years.

Ensuring that students complete their degrees or certificates should be a national priority, said Joni Finney, a professor the University of Pennsylvania’s Graduate School of Education.

“These students represent a vast resource of untapped educational capital that the country can ill afford to overlook,” Finney said in a written statement. “This report represents the most comprehensive data available of students’ attempts to navigate their path toward a college certificate or degree.”

One surprise the report found was that so many of the potential completers attended four-year institutions only, said Afet Dundar, the center’s associate director and a co-author of the report.

The same proportion -- 35.6 percent -- of potential completers attended four-year institutions exclusively as completed two-year institutions exclusively. About 30 percent attended both types of institutions.

Dundar said one might have predicted that former community college students would dominate the potential-completer category -- despite the fact that a two-year degree requires fewer credits and should take less time to finish. The reason, she said, is that community colleges enroll most of the nation’s lower-income and less academically prepared students, who are more likely to drop out of college.

That was the case for multiple-term enrollees (more than one term but less than two years). About three-quarters of this group attended two-year institutions exclusively, or before or after attending a four-year institution, the study found.

The report compared characteristics of potential completers with those who successfully earned a credential. Not surprisingly, potential completers "stopped out" -- meaning they took a break from college before returning -- more often than completers. They also spent more time along their pathway.

The center didn’t try to explain why so many Americans had short, failed stints in higher education -- such as why 10 million left after a single term or less. But the report laid out two opposing interpretations of those findings.

“This population may be viewed by some as representing the inefficiencies of a poorly aligned educational system that does not adequately prepare students with the academic skills or resources necessary to succeed,” the report said. “Others might argue that these students are a reminder of the immense opportunity offered by postsecondary institutions to students of all types, even those whose optimism and ambition may later turn out to have exceeded their determination or academic preparation.”