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Very interesting read from Justin Wolfers at the New York Times’ Freakonomics blog, who cites research from friend-of-538 Robert Erikson which suggests that polls tend to understate the performance of the incumbent.

Political scientists Robert Erikson (of Columbia) and Christopher Wlezien (of Temple) have recently mined daily polling reports from the last half-century of elections, mapping the relationship between early polling numbers and final election returns. At this point in the race, they find that around half of any lead should be discounted, as early advantages tend to dissipate. (You can read the full paper here, or an ungated version here.)

Profs. Erikson and Wlezien point to another reason to be wary of Sen. Obama’s early polling lead: On average, the voting public tends to be more strongly anti-incumbent three-and-a-half years into an administration than they are on Election Day. Based on patterns in previous cycles, the professors suggest that this exaggerated anti-incumbent feeling is boosting Sen. Obama’s lead by around three percentage points.

So if you first halve Obama’s six point lead, then subtract a three point anti-incumbency bias, you are left with a dead heat.

I just looked at this too. Although I doubt that my methodology is anywhere near as robust, I found the same thing that Erikson did: you would need to discount a polling lead more heavily for an incumbent than for a challenger.

The problem is that while McCain comes from the incumbent party, McCain himself is not an incumbent. He is not even a pseudo-incumbent (a.k.a. the sitting Vice President), the first time this situation has occurred since 1952.

Things start to get fairly complicated if you look at the interaction effects between a candidate’s lead in the polling, the number of days until the election, and the incumbency, especially since there are different degrees of incumbency, and also interaction effects and various other sorts of non-linearity between all these different variables.

But from what best I can tell, the incumbent effect that Wolfers and Erikson have identified is smaller when we’re dealing with the incumbent party rather than an actual, incumbent President — probably more like one point rather than three at this stage of the cycle. And in ’52, when you had neither an incumbent President nor a sitting Vice President running, the large lead that Dwight Eisenhower had in the polling held up quite well.

My general philosophy behind my modeling is to make everything “candidate-neutral”. The model knows that there are two candidates and that they have certain polling numbers, but it doesn’t know who is a Republican or who is a Democrat, or who is an incumbent and who isn’t. So I won’t say “the polling numbers will move toward Candidate X”, although I do say “the polling numbers are likely to move toward whichever candidate happens to be trailing”.

If I did look at that stuff, the numbers might be moved a couple points in McCain’s direction, because of this incumbent issue I just described and also because there has been some tendency for the polling to overstate the performance of the Democrat. On the other hand, if one starts to consider candidate-related variables, there are another whole set of ‘meta’ variables that one might want to evaluate too, such as the condition of the economy, the presence of a war, incumbent approval ratings, and party registration figures. Most of those factors would tend to favor Mr. Obama.

Making things candidate neutral is partially a marketing decision — I can’t imagine how much yelling and screaming there would be if I said “let’s give McCain 2 bonus points because he’s a Republican” or “let’s give Obama 2 bonus points because the economy stinks”. But also, the set of past polling data is not that robust — about 14 elections that were polled scientifically, in only the last several of which did you have multiple agencies releasing polling data at regular intervals. If you want to look at something like “Democrat challenging Republican quasi-incumbent in wartime with crappy economy”, your dataset gets down to zero pretty quickly.

Nate Silver is the founder and editor in chief of FiveThirtyEight.

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