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Researchers say online recommendation systems can be distorted by a minority of users.

By Kristina Grifantini, September 16, 2009

When searching online for a new gadget to buy or a movie to rent, many people pay close attention to the number of stars awarded by customer-reviewers on popular websites. But new research confirms what some may already suspect: those ratings can easily be swayed by a small group of highly active users.

Vassilis Kostakos, an assistant professor at the University of Madeira in Portugal and an adjunct assistant professor at Carnegie Mellon University (CMU), says that rating systems can tap into the "wisdom of the crowd" to offer useful insights, but they can also paint a distorted picture of a product if a small number of users do most of the voting. "It turns out people have very different voting patterns," he says, varying both among individuals and among communities of users.

Kostakos studied voting patterns on Amazon, the Internet Movie Database (IMDb), and the book review site BookCrossings. The research was presented last month at the 2009 IEEE International Conference on Social Computing. His team looked at hundreds of thousands of items and millions of votes across the three sites. In each case, they found that a small number of users accounted for a large number of ratings. For example, only 5 percent of active Amazon users cast votes on more than 10 products. A handful of users voted hundreds of items.

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