From whimper to tsunami, vote forecast varies

Adding to my earlier post, Nate Silver provides some pretty serious caveats to his prediction that Democrats will lose the house 233-202.  While this is the most likely outcome, his forecasting model carries a large margin of error that makes it possible for Democrats to lose anywhere from 23 to 81 seats. While the probability lies closer to the middle, just a 1- or 2-percent difference in support for either party nationwide could mean the difference between the Democrats losing the house, or holding it.

Suppose that our forecast is biased against the Democrats by one point across the country as a whole, perhaps because pollsters are overestimating the enthusiasm gap very slightly. Just one point. Well, there are 6 seats in which we have the Republican candidate projected to win by less than 1 full point (it might be a very long election night, by the way). If Democrats hold those 6 seats, the projected Republican gains would be down to 46.

 Now suppose that the forecast understates Democratic support by 2 points. There are 8 seats in which we project the Republican candidate to win by a margin of between 1 and 2 points; now these would also be wiped off the board. Now the Republican gains would be reduced to just 38 seats — and the Democrats would hold the House, 218-217!

Read that again: it means that if our forecasts turn out to be biased against Democrats by just 2 points overall, the party becomes about an even-money bet to hold the House.
Now move in the other direction. Say that we’ve underestimated Republicans’ margin by 1 point across the board. There are 8 seats that we’re currently projecting Democrats to hold by less than 1 point. Give those 8 seats to Republicans, and the gains tally grows to 60.
And if the forecast is biased against Republicans by 2 points? Another 5 seats, bringing their total to 65.

 Silver also lists the pitfalls that could befall pollsters this year:

 Consider, for instance, that the spread of recent generic ballot polls runs between a 14-point Republican advantage (Gallup’s ‘traditional’ likely-voter model) and a 3-point Democratic one (Newsweek). A mere 2-point error looks quite tame compared with that range.

Consider that one study finds that including or excluding cellphones can make a difference of 4 points in polls.

Consider that the pollsters have vastly different ideas about the magnitude of the “enthusiasm gap,” ranging from being worth just a point or two for Republicans to a double-digit advantage.

Consider that there was one recent midterm year —1998 — when the polls overestimated the standing of one party (the Republicans) by about 5 points overall, and that party underperformed their polls in virtually every individual race.

So it's not likely all these things will go awry, just as it's not likely Democrats will keep the House. But it's possible. That's all we ask.