More students than ever are taking courses online, but that doesn't mean the growth will continue indefinitely. That's the takeaway from the Sloan Foundation's latest survey, conducted with the Babson Survey Research Group, of colleges' online course offerings.

With results from nearly 4,500 institutions of all types, the report, "Online Nation: Five Years of Growth in Online Learning", found that in fall 2006, nearly 3.5 million students -- or 19.8 percent of total postsecondary enrollments -- took at least one course online. That's a 9.7-percent increase over the previous year, but growth has been slowing significantly: last year, the jump was 36.5 percent.

But compared to the growth rate for enrollment overall (1.3 percent), the report notes, the online sector is still rapidly expanding. Most of that expansion is happening where online classes are already being offered.

"The number of new institutions entering the online learning arena had definitely slowed [by last fall]; most institutions that plan to offer online education are now doing so," the report's authors wrote.

The institutions surveyed seem to believe that the most important reason for offering online courses is to improve student access, while the top cited obstacles to more widespread online offerings are student' discipline or study habits, followed by faculty acceptance.

The survey focuses solely on what it classifies as "online" courses: those offering 80 percent or more of their content over the Internet. As a result, trends in so-called "blended" or "hybrid" courses, in which students occasionally meet in person with their professors while also receiving considerable instruction online, are not covered in the report.

The importance of online courses varies widely depending on the type of institution. Public universities, for example, view online education as much more critical to their long-term strategies than private or even for-profit institutions. And not surprisingly, two-year colleges have shown the most growth, accounting for a full half of online enrollments over the past five years:

Four-Year Growth in Students Taking at Least One Online Course

The importance to online strategies is broken down in the following chart:

% Saying Online Education Is Critical to Their Institutions' Long-Term Strategy

Even if online growth can't go on at this pace forever, most institutions still see room for increasing enrollments:

% Saying They Expect Online Enrollments to Increase

Tables From "Online Nation: Five Years of Growth in Online Learning"

The study also found that most growth was expected at institutions that are the most "engaged" -- that is, "currently have online offerings and believe that online is critical to the long-term strategy of their organization. These institutions, however, have not yet included online education in their formal strategic plan."