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Former Facebook workers admit to human intervention in the choice of trending news stories Getty Images

Facebook uses a team of editors as well as its algorithms to determine the most popular news of the day in revelations that emerge as it faces accusation of bias.

The Facebook page that explains how trending topics are determined by "engagement, timeliness, pages you've liked and your location". But documents given to the Guardian reveal that in fact a small editorial team also helps decide what makes its "trending module" headlines - those topics that show up on the side of the browser window.

The staff follow guidelines including instructions on how to "inject" and "blacklist" topics in the trending topics module. They also rely heavily on only 10 sources to determine if the story has authority, including the BBC, CNN, Yahoo News and the New York Times. Other outlets it considers as having gravitas are the Guardian, USA Today, the Wall Street Journal and the Washington Post.

The technology blog Gizmodo alleged Conservative news stories had been edited out of the trending box and accusations are growing of a liberal anti-conservative bias, leading to a call for a congressional inquiry from the US Senate commerce committee chair.

In response, on 9 May, Facebook's vice president of search Tom Stocky said in a statement: "We do not insert stories artificially into trending topics, and do not instruct our reviewers to do so."

Three former editors told the Guardian they inserted stories not visible to users into the trending feed and said human choice was important, although they denied bias.

Facebook's vice-president of global operations Justin Osofsky said: "Facebook does not allow or advise our reviewers to systematically discriminate against sources of any political origin, period.

"What these guidelines show is that we've approached this responsibly and with the goal of creating a high-quality product in the hopes of delivering a meaningful experience for the people who use our service."