RealClearPolitics - HorseRaceBlog - Gallup’s Bouncing Ball
From the page:
Polls tend to have house effects, and most of us tend to notice these effects when they result in a pollster falling on one side of an average or the other. Rasmussen, for instance, tends to have Obama’s job approval on the low side while ABC News/Washington Post usually puts the President on the high side. But there are other types of house effects, and Gallup has an interesting one: it is kinda bouncy.