Tanenbaum, the founder of the poll-analysis website Electoral-vote.com, compared major pollsters' performances in the 2010 midterm Senate elections and concluded that Quinnipiac was the most accurate, with a mean error of 2.0 percent.
How accurate are Gallup surveys?
Poll analyst Nate Silver found that Gallup's results were the least accurate of the 23 major polling firms Silver analyzed, having the highest incorrect average of being 7.2 points away from the final result.
What is poll bias?
If the people who do not answer have different opinions then there is bias in the results. In terms of election polls, studies suggest that bias effects are small, but each polling firm has its own techniques for adjusting weights to minimize selection bias.