Election Update: When One Poll Makes A Big Difference

From FiveThirtyEight:

Hillary Clinton’s chances of winning the presidential election declined by nearly 3 percentage points, to 85 percent, in FiveThirtyEight’s polls-only forecast on Wednesday. Similarly, they declined by 4 percentage points, to 75 percent, in our polls-plus forecast.

The Clinton camp doesn’t really have a lot to complain about: Her position in the polls (and in our forecasts) is much improved since the Democratic convention in late July. And she’s had strong polls this week in swing states such as Pennsylvania and Wisconsin, where a Marquette University poll published on Wednesday gave her a 13-percentage-point lead over Donald Trump. So why were her numbers down in our model, on a day where there wasn’t all that much polling?

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It’s pretty much because of a single national poll conducted by Selzer & Company, on behalf of Bloomberg Politics, which gave Clinton a 4-point lead among likely voters. On the surface, that doesn’t seem all that bad for Clinton. Recent national polls have had Clinton ahead by around 7 or 8 percentage points. And we’ve had lots of national polls, so the average is pretty well established. One additional survey showing a result just a couple of points worse than the average shouldn’t be that big a deal, right?

So let me explain why this poll moved the needle, as far as FiveThirtyEight’s models are concerned. The short version: The trend line in the poll was awful for Clinton, since Selzer’s previous national polls had shown Clinton with leads of 12 percentage points in June and 18 (!) percentage points in March. For a more technical explanation, read on.

One of the most important features of our model is our trend line adjustment. Basically, the model looks at trends within individual …

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