Wednesday, October 31, 2012

National Tracking Polls vs. Full National Polls

A few days ago, I asked, "where are the national polls?" Many have pointed out that the national polls and state polls seem to be saying different things and I suggested that one possible explanation for the difference is that we weren't seeing many full national polls at all. Instead, we were awash in national tracking polls. And I questioned whether there is some difference between these full national polls and the tracking polls that could explain the disconnect between state polls and national polls.

Since then, several full national polls have come out. Let's compare these full national polls with the national tracking polls to see if they tell us something different. I've listed the last 20 national polls (full or tracking) below with the most recent dates in the field on top.

The differences here are not quite as large as they might appear at first ... but there does seem to be a little bit of a difference. If we just take a straight average of the polls on each side of the ledger, Romney leads by an average of 1.1 among the tracking polls and Obama leads by an average of 1.0 on the full national polls. But this is mostly because of a few outliers on each side. Let's throw out the biggest outlier on each side (Gallup among the national tracking polls and National Journal among the full national polls). What do we get then? Among the tracking polls, Romney still leads by 0.6 points on average and, among the full national polls, Obama leads by 0.2 points. It is a 0.8% difference.

That's not a lot but it is something. More notable than that to me is that Obama does not trail in any full national poll. It does seem to me there's something different happening with the tracking polls. If we look at just the full national polls, the split between the national vote and the state polling is just a little less large.

You probably saw that Nate Silver discussed the differences between the state polling and the national polls in a post earlier today. In that piece, he does not consider the issue I'm raising here and expresses some rationale for why he throws his lot in with the state polling. Whether what I'm talking about is a part of this story or not, Silver's argument is a good one and is good reason to think the state polling is more to be believed. But this piece may get us a small distance towards reconciling the two.

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