ProPublica, with permission, used data from GreatFire.org, a free service that anybody can use to test if a website is accessible within China. Its pseudonymous founders are activists who created the site to highlight online censorship in China.

GreatFire.org currently runs tests for the sites in ProPublica's database only upon request. Historical test results are therefore not necessarily available every day. On Nov. 17, 2014, ProPublica began initiating daily tests for 15 international news sites, and on Dec. 5, we added three more sites for testing. We plan to continue testing and updating this database every day for the foreseeable future.

On Feb. 13, 2015, ProPublica updated all of our test results to include two additional categories: "likely blocked" and "likely not blocked." "Likely blocked" includes sites where most of our test results have detected signs of censorship. "Likely not blocked" includes sites where most of our test results did not detect signs of censorship.

We chose the 18 sites in our test because of their status as internationally important news sites or because they’ve recently run stories that led them to be blocked inside China. We are able to add more sites to the database. If you think you know a site we should be testing, let us know by emailing sisi.wei@propublica.org.

The most accurate test of censorship is conducted inside the censoring country, but doing so comes with a variety of risks. Continual attempts to visit blocked sites are detectable by the local authorities, and can therefore be dangerous political activity. ProPublica has spent the last year attempting to perfect a way to test censorship around the world using computers inside each censoring country. We have not yet found a testing method that would ensure participants’ safety, though we continue to look for a solution. In the meantime, we’re using the GreatFire service already available inside China.

To show what was on each news site’s homepage when it was or was not blocked, we used PastPages.org, a service run by journalist Ben Welsh. PastPages saves screenshots of news organization homepages once every hour. It is currently tracking nearly 100 websites. Using a beta version of the PastPages API, ProPublica shows users the screenshots taken as close as possible to when we conducted each day’s censorship tests. Readers can easily explore far more screenshots of any given day on PastPages itself, using the links we provide.

Additional information on how GreatFire.org reports test results can be found on their FAQ.

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