Here is the methodology. The database contains 119 Rasmussen state polls from Jan. 1, 2012 until yesterday. For each poll, a check was made to see if at least one poll from a different nonpartisan pollster was in the data base within a week either way from the Rasmussen poll. For example, for Rasmussen’s poll of North Carolina on Oct. 2, a check was made for any other polls of North Carolina whose midpoint was between Sept. 25 and Oct 9. In this case, polls from PPP, ARG, SurveyUSA, and High Point University were found. For 82 polls, comparison polls within a week were found. For the other 37 Rasmussen polls, no other nonpartisan pollster surveyed the state within a week of Rasmussen’s poll, so those polls were not used in this analysis.
For each remaining poll, the Obama – Romney score was computed. The arithmetic mean of the other polls’ scores was then subtracted from the Rasmussen Obama – Romney value. Ideally, the result should be zero, but statistically that is very unlikely. A positive result means Rasmussen is overestimating Obama’s standing and a negative one means he is underestimating it. For example, for the North Carolina poll cited above Rasmussen said Obama was 4 points behind but the average of the other pollsters put Obama 0.2 behind, so Rasmussen gets a bias score of -3.8 here. Averaging all 82 polls, Rasmussen’s mean bias is -1.91 points, that is, Rasmussen appears to be making Obama look almost 2 points worse than the other pollsters.
As a simple example, look at the top line in the table below. on Feb. 16 (before Romney even got the nomination), Rasmussen gave Obama a 22% edge in California, but another poll within a week said Obama was 20% ahead. In this case Rasmussen has a positive bias (for Obama). On the line below, Rasmussen’s bias is -2%, that is, against Obama.