To read the most recent posts by Nate Silver and Sam Wang regarding their predictive models vis a vis the November midterms, you might think the main event on November 4 is between them. It’s an entertaining dispute, no doubt about that and the two wizards are having fun and getting plenty of attention from political data junkies.
Here’s the latest from Wang’s “Is Nate Silver a little too excited about his model?” in a couple of nut graphs:
The PEC Election Day prediction indicates a 70% probability of Democratic+Independent control. That is based on polls alone, plus the assumption that September-October will act like June-August. FiveThirtyEight’s probability is 64% favoring the Republicans, based on a model with polls plus a substantial dose of special sauce (a.k.a. fundamentals).
…I have to say, this special sauce is messy stuff. Really, the GOP has an 25% chance (3-1 odds) of getting 54 or more seats? I’d put it at more like 5%. Even 53 GOP seats is a fairly outside outcome. If a betting person were offered the chance to put up $3000 against Nate Silver’s $1000 on that outcome…that would be taking his money.
Joking aside, there are two serious points to be made here. First, nobody should be getting excited about any probability that is in the 20-80% range. That includes Nate Silver, who should knows better. He must need the media attention. Second, the addition of “fundamentals” and other factors adds considerable uncertainty to the projection.
Meanwhile, over at FiveThirtyEightPolitics, Silver’s “Registered Voter Polls Will (Usually) Overrate Democrats” adds this to the donnybrook:
Polls of so-called likely voters are almost always more favorable to Republicans than those that survey the broader sample of all registered voters or all American adults. Likely voter polls also tend to provide more reliable predictions of election results, especially in midterm years. Whereas polls of all registered voters or all adults usually overstate the performance of Democratic candidates, polls of likely voters have had almost no long-term bias….We can infer that, because likely voter polls have no long-term bias and registered voter polls show more favorable results for Democrats, registered voter polls usually have a Democratic bias.
…Likely voter polls have been unbiased, whereas registered voter polls have had a median Democratic bias of 2 percentage points.
That’s why our model adjusts registered voter polls in the way it does; their Democratic bias is fairly predictable, especially in midterm years.
Silver links to an informative chart at The Upshot, comparing forecaster snapshots of senate races, which show a slight Republican edge. But Wang isn’t in it, and he still ain’t having it.
There’s more to Silver’s analysis, and yes, it’s complicated. Of course Dems hope Wang is right. But Silver is extremely cautious in his assumptions, and his track record is pretty damn good, so who knows?
There’s room enough here for either Wang or Silver to be wrong when all the votes are counted. But neither one of them would argue with the conclusion that it’s high time for Democrats to pour on the GOTV in the battleground states on an unprecedented scale. That simply has to happen if Dems want to keep a senate majority.