It's one of the most hyped ideas in higher education today — the hope that college courses taught online can drive down the cost of a degree, and make it easier for working students to complete their college education.

But a new study comparing the success rates of online and traditional, face-to-face courses taught at Washington's community colleges shows more students drop out, and fewer get a passing grade, when they take a class online than when they take it in a classroom.

And the students who fare the worst are those who are already struggling in college, raising the possibility that a push to more online classes could exacerbate the higher-education achievement gap.

The study was done by the Community College Research Center at Teachers College, Columbia University, at the request of Washington's community colleges.

The data included results from 500,000 courses taken by 40,000 students over four years -- tapping into "a strong and robust data system" the state keeps on success rates in community colleges, said Shanna Smith Jaggars, assistant director of the research center.

Smith Jaggars, who was one of the study's authors, said many previous studies comparing online and in-class course success have been limited in scope, usually only comparing the results of a single course taught in-person at a four-year college to the same course taught online.

Those studies often show that online is just as good as in-person, she said.

But the study of Washington's community colleges -- and a comparable study of Virginia's -- is much broader because it analyzes a whole system over an extended time period, she said.

It found that males, black students and students with lower levels of academic preparation were the ones most likely to fail to finish a class, or get a lower grade. Those groups already have lower performance levels in college, and the gaps worsened in online courses.

"If this pattern holds true across other states and educational sectors, it would imply that the continued expansion of online learning could strengthen, rather than ameliorate, educational inequity," the study says.

In the Virginia study, "the main thing that was really frustrating about online courses is they (students) found it difficult to connect with the instructor," Smith Jaggars said. "Students said to us that they often felt like they were teaching themselves."

And even though technology has evolved in recent years, the online class success rate has not improved, Smith Jaggars said.