From Nate Silver’s FiveThirtyEight blog on NYTimes.com:

Public Policy Polling conducts surveys for Democratic candidates (and Daily Kos, a liberal blog) in addition to issuing surveys under its own name. Until recently, we have not found an especially large “house effect” for Public Policy Polling — that is, they’ve had plenty of surveys showing poor numbers for Democrats. But lately, such an effect has arguably become more noticeable: they are the only pollster, for instance, to show the Democrat Michael Bennet with a lead in Colorado, although several other pollsters have shown that race tightening. And their survey of Arizona’s Third Congressional District, which showed the Republican Ben Quayle trailing in a district that ordinarily leans Republican, has raised a few eyebrows, although Mr. Quayle may be an unappealing enough candidate that the result is not necessarily implausible.