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Nate Silver, FiveThirtyEight.com, April 17, 2010The bottom line is this: the sample included in
Rasmussen's polling is increasingly out of balance with that observed by almost all other pollsters. This appears to create a substantial house effect, irrespective of whether Rasmussen subsequently applies a likely voter screen.
It also appears to be a relatively new facet of their polling. If one looks at the partisan identification among all adults in polls conducted in September-November 2008, Rasmussen gave the Democrats at 6.5-point edge, versus an average of 8.7 points for the other pollsters; their house effect was marginal if there was one at all.
Techniques like weighting can correct for some of this response bias, but it can be an imperfect defense, particularly for pollsters like
Rasmussen who have very low response rates (because of their "flash" one-night samples and their use of IVR technology).
If, on the other hand, this is a feature rather than a bug, it requires a more robust explanation from Rasmussen.
It is not sufficient, after all, to believe that Rasmussen is getting it right: you also have to believe that almost everyone else is getting it wrong.Their use of a likely voter model alone is not sufficient to explain the differences.
Citing Rasmussen's success in calling past election outcomes, which is formidable, is also somewhat non-responsive, since their house effect was not so substantial in past election cycles.