As the demand for online education grows, so does the number of companies that brand themselves as simple and effective teachers. Among the latest is SpacedEd.com, a Web site based on research developed by a professor at Harvard Medical School.

Launched in May, SpacedEd allows users to develop courses on topics ranging from "Core Cardiology for Medical Students" to "Bartending 101." Enrolled students receive a set of questions as frequently as once a day, via e-mail or RSS feed (text messaging and instant messaging are in the works). All the teaching is done through a trial-and-error testing method: Answer a question wrong and it repeats; get it right and it repeats less often; get it right multiple times and it disappears. An algorithm adjusts for the student's level and content knowledge, based on his or her score as it develops.

One question from "Core Anatomy for Medical Students" reads: "A patient with long-standing poorly-controlled diabetes can no longer plantarflex her right foot but can still evert it. The nerve that is most likely affected by her diabetic neuropathy is the ...". Possible answers: the sciatic nerve, superficial peroneal nerve, deep peroneal nerve and tibial nerve. Should a student choose incorrectly, the following page reveals the correct answer (the tibial nerve), explanations and related links, as well as a note that the question will repeat in the future.

The research behind SpacedEd was developed by B. Price Kerfoot, an associate professor of surgery at Harvard Medical School. His previous research had demonstrated that people are more likely to retain information when they see it periodically and are tested on their knowledge, rather than simply provided with reading material.

Kerfoot said he has tested the method in more than a dozen trials with 7,000 medical students, doctors and other participants. Overall results have indicated that it helped them boost their knowledge and retain it for up to two years. In one study, two randomly divided groups of 240 physicians each took a course about clinical practice guidelines in urology. One group was presented a section of the material three times at spaced intervals over 20 weeks. By the end, that group demonstrated a 50 percent increase in knowledge of that material compared with the control group.

Kerfoot said that the method has also proved to be an effective teaching tool for medical students from Baylor University, the University of Pittsburgh, Harvard and several other institutions. Beyond medicine, Kerfoot said its testing method can be applied to fields across higher education.

SpacedEd currently has more than 2,000 users and two dozen courses, all of them free (though authors can charge and earn a slice of the revenue) and none of them for academic credit. Any registered user can construct lessons about anything, including non-science subjects like bartending, music theory and copyright law. Site administrators rely on users' comments to check for accuracy and clarity, Kerfoot said.

"Core Anatomy for Medical Students" was designed by Kitt Shaffer, vice chair for education in the Department of Radiology at Boston Medical Center. A colleague of Kerfoot, she said she does not usually use online education systems, but likes being able to embed images and audio on SpacedEd. Above all, she said, the site simplifies the learning process.

One of Shaffer's students at Boston University School of Medicine, Rob Gordon, recently signed up for her anatomy course and is planning to develop a radiology course."Each day, I think about what I read and learned and see the same questions over again," he said. "I think it is a great supplemental way of learning."

Web-based learning, which is gaining momentum among colleges and universities, got a boost from a report released by the U.S. Department of Education in June. The study found that students who received their instruction online performed better, on average, than those who took the same class through face-to-face interaction. Those who took courses that combined online learning and face-to-face instruction performed best of all .

The technology based on Kerfoot's research is patented by Harvard. SpacedEd is likely the first online educational-product company Harvard has patented, said Daniel Behr, the university's director of business development. He did not disclose the costs involved in the licensing agreement, but said Harvard will benefit financially; most of the revenue will be funneled into research efforts.

John Bourne, executive director of the Sloan Consortium, a group of colleges and other organizations that promotes online learning in higher education, tried some of the courses on SpacedEd. Overall, he said he was not totally impressed.

"It's difficult to really characterize this as an online learning system," he said. "There's very little activity in it in which you interact with people or talk about things." He added that the site's question-and-answer format seemed "good for facts and less good for discussion"; that is, more conducive to teaching science rather than the humanities.

"It's an interesting start," he said. "But at first blush, it doesn't seem to me to be a whole lot different from computer-aided instruction."