Don't Count Your Models Before They've Hatched
Ezra Klein touts a new statistical model forecasting "an expected Democratic gain of 32 seats with Democratic control (a gain of 18 seats or more) a near certainty." Ezra remarks, "All the usual disclaimers apply, but things would have to go mighty awry for this election to slip through the left's fingers."
Well, let me offer some disclaimers. One is that there are two kinds of models based on historical data. One kind looks at the historical data, devises a model that fits the historical data well, and then offers a prediction based on the model. That's what these guys have done. In another kind of model, you do the same thing and then, having offered your prediction, you wait for the election outcome and it turns out that your prediction was good. Then, next time around, the same thing happens. That is to say that in the second sort of situation your model is not only based on historical data, but has an actual track record of success. I'd be a lot more confident in a model with a track record, since there are actually any number of formulae that might fit the historical data well.
More concretely, I still worry that we might see a new al-Qaeda video aimed at tipping things toward the GOP. I wish more liberals were out there putting this worry and Brad Plumer's argument about it, out there before it happens.