A recently released study by U.S.-based computer scientists surveys censorship on the popular Chinese social networking platform Sina Weibo. A summary of the study by MIT Technology Review explains the methodology used to conduct research on the popular microblogging platform:

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, it’s 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.

Tech In Asia’s Charlie Custer sifted through the report, drawing quotes that show just how fast Sina’s internal censors work:

The study, which we came across via MIT Technology Review, used “architecture [that could] detect post deletions within one minute of the deletion event,” giving the researchers perhaps the most precise look yet into how quickly Sina’s content team takes down sensitive Weibo posts. The results? Sina is pretty darn fast:

We found that deletions happen most heavily in the first hour after a post has been submitted. Focusing on original posts, not reposts/retweets, we observed that nearly 30% of the total deletion events occur within 5-30 minutes. Nearly 90% of the deletions happen within the first 24 hours.

So Sina’s censors are pretty fast. But what, exactly, are they deleting? Researchers used a variety of analytical tools to look at what content was most quickly deleted, and found that:

The topics where mass removal happens the fastest are those that combine events that are hot topics in Weibo as a whole (e.g., the Beijing rainstorms or a sex scandal) with themes common to sensitive posts (e.g., Beijing, government, China, and policeman).

A post at Quartz notes that the study seems to have discovered when censors are likely to take a break:

Here’s a tip for the 300 million users of China’s Twitter-like microblogging site Sina Weibo: If you don’t want to get censored, post your messages at around 7pm, Beijing-time.

[…]These censors are monitoring and deleting posts around the clock, often very late at night, the study notes. But there are two noted dips: one at around 7pm, when the national news is on, and again early in the morning, when most of the country is asleep. You can see that trend in this chart from the study [click through].

For more on Sina Weibo or Internet censorship, see prior CDT coverage.