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The study is the first large-scale analysis of political content censorship in social media, a topic that drew attention and controversy earlier this year when Twitter announced a country-by-country policy for removing tweets that don't comply with local laws.

In China, where online censorship is highly developed, the researchers found that oft-censored terms included well-known hot buttons, such as Falun Gong, a spiritual movement banned by the Chinese government, and human rights activists Ai Weiwei and Liu Xiaobo. Others varied based on events; Lianghui, a term that normally refers to a joint meeting of China's parliament and its political advisory body, became subject to censorship when it emerged as a code word for "planned protest" during pro-democracy unrest that began in February 2011.

The CMU study also showed high rates of weibo censorship in certain provinces. The phenomenon was particularly notable in Tibet, a hotbed of political unrest, where up to 53 percent of locally generated microblogs were deleted.

The study by Noah Smith, associate professor in the Language Technologies Institute (LTI); David Bamman, a Ph.D. student in LTI; and Brendan O'Connor, a Ph.D. student in the Machine Learning Department, appears in the March issue of First Monday, a peer-reviewed, online journal.

"A lot of studies have focused on censorship that blocks access to Internet sites, but the practice of deleting individual messages is not yet well understood," Smith said. "The rise of domestic Chinese microblogging sites has provided a unique opportunity to systematically study content censorship in detail."

The so-called Great Firewall of China, which prevents Chinese residents from accessing foreign websites such as Google and Facebook, is China's best known censorship tool. Other countries also are known to block Web access, such as when Egypt shut down Twitter and other social media sites during last year's Arab Spring protests.

But blocking access to all sites and services is impossible if China or any other country is to harness the Web's commercial and educational potential, Bamman said. An alternative is to allow access to sites, but police the content, eliminating messages deemed objectionable. Automated methods may be used to eliminate some messages, while others are deleted manually, he noted. Seldom are all weibos with a sensitive term deleted, but anecdotal evidence is overwhelming that certain messages are targeted.

"You even see some weibos where the writer asks, 'Is this going to be deleted?'" O'Connor said. In late 2010, New York Times columnist Nicholas Kristof opened an account on a Chinese microblog site; within an hour of sending a message about Falun Gong, his account was shut down.

To study this "soft" censorship, the CMU team analyzed almost 57 million messages posted on Sina Weibo, a domestic Chinese microblog site similar to Twitter that has more than 200 million users. They collected samples of weibos from June 27 to Sept. 30, 2011, using an application programming interface (API) that Sina Weibo provides to developers so they can build related services.

Using the same API, they later checked a random subset of weibos to see if they still existed and another subset that included terms known to be politically sensitive. If a weibo was deleted, Sina would return what the researchers came to regard as an ominous message: "target weibo does not exist."

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First large-scale analysis of 'soft' censorship of social media in China