This is primary research by AllSides - see About Bias to learn more about the patented AllSides Bias Rating(tm) (ABR) system.
This particular blind survey used headlines from one particular day in October 2013. Participants viewed the top 5 headlines along with the beginning portions of each story for several different news sources, and based on that alone they rated the bias of each source. Note they could not see where the headlines were from - they did not know the sources.
Here is a graphical example, this one is for Bloomberg which was moved from Lean Right to Center because of these results: