Research has repeatedly shown that high school dual enrollment correlates with higher rates of college attendance and graduation. But it’s unclear how different ways of delivering such courses—online or in person, by high school teachers or college faculty, in a high school or on a college campus—impact their effectiveness.

A new study out of Kentucky, where 46.2 percent of high schoolers graduate with dual-enrollment credits, is among the first to dig into the modalities that lead to best outcomes for students. Looking at five years of state dual-enrollment data and controlling for factors like past academic success, researchers found that courses taught by high school teachers in person on a college campus were the most effective; taking just one such course boosted a student’s likelihood of enrolling in college by 11 percentage points compared to those who took no dual-enrollment courses. By comparison, taking a single dual-enrollment class in any modality increased a student’s likelihood of college enrollment by five percentage points.

Taking a college-level course at the high school, whether taught by a college faculty member or a high school instructor, correlated with a seven-percentage-point increase. Students gained a smaller boost from taking a class taught in person by faculty at a college (two percentage points), and were no more likely to enroll in college after taking an online course taught by college faculty than if they’d taken none.

The research, which hasn’t been published, did not delve into why, exactly, some modalities are better at encouraging students to enroll in higher education than others. But Christopher Ledford, director of data and advanced analytics for the Kentucky Council on Postsecondary Education, speculated—in part based on his own past experience as a dually enrolled student—that programs that combine elements of both high school and college serve to expose students to college-level schoolwork without completely overwhelming them.

“For me, that familiarity … but also that sort of, ‘Hey, this is a college class, I made it, I can do this’—those blended elements really helped me be successful,” he said.

According to Alex Perry, senior education policy adviser at Foresight Law + Policy and the coordinator of the College in High School Alliance, blended modalities are significantly less common than online courses taught by faculty or college courses taught at the high school by high school instructors. This is in large part due to difficulties transporting students to college campuses, aligning schedules between the two institutions and even getting food to the high school students who rely on free school lunches while they’re on the college campus.

“I’ve heard cases of the high schools driving up and handing a bag lunch out the window, because otherwise the students just wouldn’t eat during the day if they’re on the college campus,” he said.

While it’s difficult to create policies to promote programs that blend elements of both, he added, states could provide incentives to high schools and colleges that encourage them to align their schedules.

It’s also unclear why online courses are proving less effective. Perry noted that forthcoming research from the Early College Research Center, which will explore the same modality questions as the Kentucky study, shows more positive outcomes for online dual-enrollment courses, indicating variability by state.

And just because online courses don’t boost a student’s college-going likelihood by a significant margin doesn’t mean that that modality, which is the most convenient for a large number of students, should be abandoned, argued Robin Hebert, senior fellow for student access and success at the Kentucky Council on Postsecondary Education.

“A big question is, we know the online courses are a bit flat—what can we do to improve that? Because I think the online modality is really important in terms of access,” she said. “We want to make sure that all of the students in Kentucky can get access to dual enrollment. So, instead of saying, ‘Well, we need to have less of that,’ maybe we just need to figure out how to make that give us the results that we’re after.”