NewsSniffer monitors The Independent, Guardian Unlimited and BBC News throughout the day to check for changes made to published stories. Revisions are stored and highlighted for comparison by users, who are then invited to rank the most intriguing differences.
The site's Revisionista tool illustrates the evolution of articles, including corrections to spelling and grammar mistakes. Its Watch Your Mouth feature stores comments "censored" after publication in BBC News' 'Have Your Say' threads.
"We're looking for systemic bias," Leeds-based software developer John Leach, the man behind the site, told Journalism.co.uk. "I'm of the belief that the corporate media has a pro-corporate agenda so I expect to find bias towards this.
"BBC, The Guardian and The Independent are all generally considered the 'liberal' media here in the UK, but all have a very pro-corporate agenda due to relying completely on the free market."
Mr Leach said he started the site after hearing complaints from Have Your Say readers who could not fathom why their comments had been removed from the BBC discussion area.
Writing online, he admitted his "design spec assumed a sneaky BBC" and he said he plans to release the code behind his site so readers can monitor news sources using their own sniffer.
The tool could now attract media watchers and others keen to monitor fairness and balance, such as those concerned with the Israeli-Palestinian conflict or with election reporting.
But it still includes several bugs and may bring suspicion on revisions that are merely breaking news updates or discussion threads flagged for removal by readers themselves under Have Your Say's recently introduced self-moderation facility.
"Looking through the examples offered, there doesn't appear to be much that's sinister there; bias is often in the eye of the beholder," wrote the BBC's director of global news, Richard Sambrook.
"I actually think it's a good idea and no news organisation should be worried about being held to account for changes made - usually it's for clarification or accuracy or of little consequence. And when it may be of some consequence news organisations should be prepared to explain."
Hackers have made previous attempts to document daily news patterns. A script written by web accessibility enthusiast Matthew Sommerville in 2005 shows minute-by-minute updates to BBC News' front page, while a system similar to NewsSniffer's Revisionista is said to have been developed earlier in-house at the BBC.
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