Carnegie Mellon University researchers have analyzed millions of Chinese microblogs to discover exactly what terms are being censored.
Some of the worst-offending names include Falun Gong, a spiritual movement banned by the Chinese government, and human rights activists Ai Weiwei and Liu Xiaobo.
Others are less obvious: 'lianghui', a term that normally refers to a joint meeting of China's parliament and its political advisory body, emerged last year as a code word for 'planned protest'.
The microblogs, or weibos, are being deleted at different rates depending on the area of the country, says the team. This was particularly notable in Tibet, where up to 53 percent of locally generated microblogs were deleted.
"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," says associate professor Noah Smith.
"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. But China takes a micromanagement approach too, eliminating certain individual messages.
"You even see some weibos where the writer asks, 'Is this going to be deleted?'" says PhD stiudent Brendan O'Connor.
The CMU team analyzed almost 57 million messages posted on Sina Weibo, a domestic site similar to Twitter, with more than 200 million users. They collected samples of weibos from June 27 to September 30, 2011, using an API that Sina Weibo provides to developers so they can build related services.
They later checked a random subset of weibos to see if they still existed.
Not all the censored terms were political, they found. Following the March 2011 Fukushima nuclear disaster in Japan, weibos containing such politically innocuous terms as 'iodized salt' and 'radioactive iodine' had high deletion rates.
The researchers believe these deletions were caused by government efforts to quash false rumors about the nuclear accident causing salt contamination.