In a July 13 article, Pollster.com released an analysis of Rasmussen Polls, suggesting that Rasmussen's results have a pro-Republican bent:

Whenever I meet a Democrat, and they find out I write for Pollster.com, the first question almost always asked is “what are your thoughts on Rasmussen? They are always biased against the Democrats”. On the liberal site Daily Kos, Steve Singiser's nightly political wrap-up packs all Rasmussen polls at the bottom of his posts and mockingly refers to the polls as from the “House of Ras”. I also got in on the criticism last month in a post where I pointed out that Rasmussen had higher pro-Republican house effects during important news cycles in 2008.

But does Rasmussen have a large pro-Republican house effect in 2010? Looking at the generic ballot, the answer seems to be absolutely. Using only Rasmussen polling, the Pollster.com aggregate gives the House Republicans a 7.3% advantage as of this writing. Using all other pollsters except for Rasmussen, House Republicans hold only a 0.1% lead. Some of this may be their use of a likely voter model, although it is unlikely that it accounts for all of Rasmussen's difference.

Of course, the generic ballot is only one of the many contests that Rasmussen polls. Rasmussen has accounted for a little over 50% of the legitimate polls conducted for United States Senate races in 2010. Being that Rasmussen has flooded the zone, we must ask whether their Senate polls have had the same sort of house effect as their generic ballot. Many liberals would like to believe so, but no one to my knowledge has affirmed or disproved it... until now.

David Shor, a visiting graduate student collaborator at Princeton University, has estimated Rasmussen's house effects in all possible Senate races (his methods are outlined here). Using data supplied and collected by Rasmus Pianowski and me, he found that the difference in house effects between Senate races to be mostly insignificant (or “eerily consistent”) . However, the difference between Senate races and the generic ballot was highly significant*.

Instead of a 5% pro-Republican house effect as seen on the two way generic ballot, the pooled Senate house effect is only 2%. For example, if you encounter a 44% to 36% Republican lead on Rasmussen's generic ballot, it is probably a tie: 44% for the Republicans / (44% for the Republicans + 36% for the Democrats) - 5% House Effect = 50% of the decided vote for the Republicans. On the other hand, a 44% to 36% Republican lead in a Senate race means the Republican is leading: 44% for the Republican / (44% for the Republican + 36% for the Democrat) - 2% House Effect = 53% for the Republican of the decided vote.

Does this variation in generic vs. Senate house effects make a difference in terms of how many seats Republicans would pickup if the election were held today? Shor, who, along with his Stochastic Democracy blog team, Princeton Election Consortium, and me launched a preliminary 2010 Election Projection System, has shown that differing house effects do make a difference in our projections. If the Rasmussen house effect from the generic ballot had been applied to Senate races, Republicans would only be predicted to pick up 3 seats. With a Senate specific house effect, they are predicted to pick up 5 seats with Missouri and Pennsylvania falling into the Republican column.

It should be pointed out that Rasmussen, like any other pollster, will have outlier polls that will not fit perfectly in with any assigned house effect. This analysis also tells us that on average some of the pollsters in this chart had consistently different results that benefited one party's candidates. More specifically, when viewing Rasmussen's Senate polls, one should realize that these polls tend to be more pro-Republican than other polls, but not as pro-Republican as Rasmussen's generic ballot polls. Of course, we will not know whether this house effect portends to accuracy until Election Day.

*Note: House effects were estimated in each Senate race and then averaged to create a single “Senate house effect”. In the case of YouGov and Quinnipiac, the Senate and generic ballot house effects were averaged.