The AllSides Bias Rating™ currently gives HuffPost (formerly The Huffington Post) a media bias rating of left. Over half of the 16,000 AllSides users who voted disagree, but note that this raw voting data from AllSides users is not normalized (so most votes could come from one political segment or from an outside group hoping to influence our ratings). The results from the January 2013, March 2013, and October 2013 Blind Surveys (which are normalized to better reflect American opinion) clearly indicate that The Huffington Post has a left bias (see other highlights of that study here). The Huffington Post identifies itself as an overtly liberal news and information site, which is consistent with the AllSides blind bias rating studies.
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HuffPost is a liberal online news source founded by Arianna Huffington in May of 2005. Formerly The Huffington Post, the news and blog site is one of the most-read and cited sources online. Huffington won the Pulitzer Prize in 2012 for national reporting. She is part of Time Magazine’s list of the world’s 100 most influential people and the list of Forbes most Powerful Women. According to the Alexa Global Traffic Rank, The Huffington Post was ranked as the number one most popular political site in July of 2012.
Wikipedia: The Huffington Post