Roughly one in six students enrolled in higher education — about 3.2 million people — took at least one online course last fall, a sharp increase defying predictions that online learning growth is leveling off.

A new report scheduled for released Thursday by The Sloan Consortium, a group of colleges pursuing online programs, estimates that 850,000 more students took online courses in the fall of 2005 than the year before, an increase of nearly 40 percent.

Last year, the group had reported slowing growth, prompting speculation the trend had hit a ceiling.

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"The growth was phenomenal," said Jeff Seaman, Sloan's CIO and survey director, who also serves as co-director of the Babson College survey research group. "It's higher in absolute numbers and higher in percentages than anything we've measured before. And it's across the board," at schools ranging from doctoral institutions to those offering associate's degrees to for-profit colleges.

Some online programs have flopped, and several for-profit universities have seen their share prices slump in the last two years amid concern over online's growth prospects.

Shares of Apollo Group (APOL), which owns the giant for-profit University of Phoenix and is now embroiled in a stock-option scandal, are more than half off their 52-week high.

Still, many universities are investing heavily in online learning, hoping the model will prove more economical than traditional classes, thus expanding their reach.

A recent survey by Eduventures, a consulting and research firm, found 50 percent of consumers who expected to enroll in a higher education program said they would prefer to get at least some of their instruction online.

About 80 percent of online students are undergraduates, and they are generally older and more likely to be working and have families. But only about half are pursuing online degrees, according to Eduventures.

The rest are taking individual online courses or — increasingly — mixing online courses with more traditional campus-based classes.

One reason online enrollment may be growing is that the difference between traditional and online classes is blurring.

It's not unusual now for traditional classes to post syllibi and homework assignments online or to have class discussions in group forums. Some classes take place more than 80 percent online, which makes them count as online courses for the Sloan survey.

"That's bumping up enrollment," said Eduventures senior analyst Richard Garrett.

The Sloan survey results also suggest academic officials are becoming more comfortable with online learning. About 62 percent of chief academic officers said they felt students learned as well or better from online courses as they did in face-to-face ones.

However, that left about 38 percent who found online courses degraded the educational experience. And almost all said they aren't certain online learning will be more widely adopted.

Among the obstacles: online courses take more time and effort to prepare, students need more self-discipline, and faculty often aren't convinced online learning is worthwhile.

Officials at the schools surveyed "all acknowledge that there are significant barriers," Seaman said. "The question is going to be when do those barriers kick in and how do they cope with them."