Computer Scientists Measure the Speed of Censorship On China’s Twitter

Censorship on Weibo, Chinas version of Twitter, is near real-time and relies on a workforce of over 4,000 censors who stop work during the evening news, according the first detailed analysis of censorship patterns.

The Chinese version of Twitter is a microblogging service called Weibo which launched in 2010. This allows users to post 140 character messages with @usernames and #hashtags, just like Twitter although 140 characters in Chinese contain significantly more information content than in English.

In just three years, Weibo has picked up some 300 million users who between them send 100 million messages each day at the rate of 70,000 per minute. That makes the inevitable process of censorship a tricky task for the Chinese authorities. So an interesting question is how they do it.

Today,Dan Wallach at Rice University in Houston, Texas, and a few pals reveal the results of a detailed study of censorship on Weibo. Their method has allowed them to reconstruct the censorship techniques used by the government, to calculate the number of workers who must be involved and even to discover their daily work schedules.

The work is possible because at least some of the content on Weibo is not censored prior to publication, only afterwards. Their approach was to collect posts from a set of users once every minute. They then tracked these posts to see which ones later became unavailable.

Of course, its not feasible to track everyone on Weibo so Wallach and co spent some time looking for users who seemed to have posts deleted more often than others, assuming that these users would be more likely to be censored in the future. Using this manual technique, they ended up observing some 3500 users over a period of 15 days last year who between them experienced around 4500 deletions per day, or about 12 per cent of the total.

Not all deletions are the result of censorship, however, since a user can delete his or her own posts. Wallach and co say that through their own trial and error they observed two types of deletion which return different messages. When users delete their own messages, a query for the post returns a post does not exist error message.

However, when a post is deleted by the censors, Weibo returns a different message saying: permission denied. It is these second type of deletions that Wallach and co concentrated on.

The results of their study are fascinating. They say that in their data set about 5 per cent of the deletions occur within 8 minutes of posting and around 30 per cent within 0 minutes. In total, 90 per cent of deletions occur within a day, although at times deletions can occur several days later.

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Computer Scientists Measure the Speed of Censorship On China's Twitter