Media Matters bias rating is Left. AllSides provides this rating with High confidence.
As of May 2017, a majority of AllSides users who voted on this bias rating agreed. Of those who disagree, the average rating falls on the cusp of a Lean Left and Center bias rating. There is not enough evidence to change the rating at this time. Furthermore, on Media Matter's website, they define themselves as a "progressive research and information center dedicated to comprehensively monitoring, analyzing, and correcting conservative misinformation in the U.S. media."
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Media Matters for America is a 501(c)(3), web-based, not-for-profit organization. Media Matters was founded in 2004 by journalist and activist David Brock to "systematically monitor a cross section of print, broadcast, cable, radio, and Internet media outlets for conservative misinformation...every day, in real time." They publish research and analytic reports as well as rapid-response items and try to keep not only activists, but also the general public aware of misinformation. Some people consider Media Matters to be the media watchdog counterpart to the more conservative Media Research Center. They are also known for having declared a "War on Fox" in 2010.