There are dozens of presidential opinion polls these days, and even more interpretations what they mean. But predictions keep changing, which calls into question how useful they are. The closer you get to an election the easier it is to call it, at least in theory. But it is not really predicting when you change your take on the race with every new poll. For example Nate Silver at his New York Times blog has consistently “predicted” an Obama victory, yet has altered the odds on a daily basis. This is the equivalent of claiming you can predict the outcome...