Villeneuve found 313 websites that were censored by at least one of the search engines during at least one of the tests he conducted. However, only 76 were censored at least once by all four, and, of those, only 8 were censored by all four each time he tested. Google had the lowest average number of censored sites, at 15.2 percent of those tested. Microsoft censored 15.7 percent of sites, followed by Yahoo at 20.8 percent and Baidu at 26.4 percent.

Another characteristic Villeneuve tested was transparency, meaning how clearly a search engine notifies a user that a result has been censored. He found that Google maintained the highest transparency, while Microsoft and Yahoo both had slightly less transparency than they did in 2006. Though Microsoft said in a statement that it is committed to providing notification, Villeneuve found that such notices occurred only with general keyword searches, not searches targeted to specific sites. He notes that Yahoo's notifications, which accompany any search, whether its results have been censored or not, makes it difficult to determine which sites have been censored and which were simply not indexed.

"The question in China is made very complex in that there is a combination of tacit government guidance in what one should block, and there's also a lot of guessing by the companies," says Derek Bambauer, an assistant professor of law at Wayne State University. "These factors come together to lead to overblocking." Bambauer says the Citizen Lab's report is important because it provides rigorous methods that can be used to compare the companies' censorship practices.

John Palfrey, executive director of the Berkman Center for Internet and Society, says, "Search companies are plainly conservative about making decisions of what to block because the law is so unclear." He adds that the "biggest thing companies can do is to work together on a common front," a situation that could potentially reduce overblocking.

Villeneuve says that he thinks it's important to independently monitor the search companies so that there's pressure for them to remain accountable for their commitments to minimizing censorship. Going forward, he says, he would like to see companies post clearer notices about what is being blocked and why, perhaps by citing which laws specifically are making content unavailable.