For a complete methodological discussion, see here. I'm just going to reiterate just a few very high-level bullet points.
-- These ratings pertain to just one particular type of poll: those for which the field work was conducted within the 21 days preceding a public election, which surveyed people about their voting intention in that election, and which was released into the public domain in advance of the election.**
-- These ratings reflect polling for President (general and primary elections), U.S. Senate, U.S. House, and gubernatorial races since 1998. More recent cycles are weighted more heavily. This is a truly massive amount of data: roughly 4,700 polls.
-- The variable called rawscore is the most direct measure of a pollster's track record. However, it is much inferior to PIE -- or Pollster-Introduced Error -- for evaluating the effectiveness of different pollsters on a going-forward basis. Because polling involves a great deal of luck in the near-term, rawscores must be substantially regressed toward the mean. However, different types of pollsters are regressed to different means. In particular, pollsters that have made a commitment to transparency and disclosure have been shown to have superior results over the long-run. The way we measure this is whether the pollster was a member of either the NCPP or the AAPOR Transparency Initiative as of 6/1/10.
-- PIE is expressed as a positive number and reflects the amount of error that a pollster introduces above and beyond that which is unavoidable due to things like sampling variance. The lower a firm's PIE the better.
The list below provides ratings for all firms with a minimum of 10 polls. A complete list of ratings (including for firms with fewer than 10 polls) follows it below the fold.
Ratings for firms with at least 10 polls
Ratings for all firms (no minimum number of polls)
** Correction: This language previously read: "These ratings pertain to just one particular type of poll: those which attempt to forecast election outcomes, and do so in the public domain." This had been improperly stated. This exercise accounts for all polls released into the public domain, whether or not it was the pollster's intention to forecast the election outcome. Some polling organizations do not regard their polls as constituting "predictions" or "forecasts", and do not recommend that their polls be used for forecasting purposes.