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Here are the latest forecast probabilities on FiveThirtyEight.

FiveThirtyEight    Clinton    Trump
---------------    -------    -----
Polls Only            51.0     49.0
Now Cast              56.8     43.2

What explains the difference between the Polls-Only forecast and the Now-Case forecast?

  • I was about to ask the same question. A few days ago, the now-cast had Trump at a 10pt lead, it's since flipped, and now, just 21 hours after your post, it's 63.6/36.4, a large further jump, in my opinion. – JTP - Apologise to Monica Aug 1 '16 at 14:39
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The numbers you're comparing are not "poll results", they're odds of winning based on the polling.

The details of their models are spelled out here. They summarize them as:

Polls-plus: Combines polls with an economic index. Since the economic index implies that this election should be a tossup, it assumes the race will tighten somewhat.
Polls-only: A simpler, what-you-see-is-what-you-get version of the model. It assumes current polls reflect the best forecast for November, although with a lot of uncertainty.
Now-cast: A projection of what would happen in a hypothetical election held today. Much more aggressive than the other models.

Then, they even have a section called "Differences between polls-only and now-cast":

  • The now-cast is basically the polls-only model, except that we lie to our computer and tell it the election is today.
  • As a result, the now-cast is very aggressive. It’s much more confident than polls-plus or polls-only; it weights recent polls more heavily and is more aggressive in calculating a trend line.
  • There could be some big differences around the conventions. The polls-only and polls-plus models discount polls taken just after the conventions, whereas the now-cast will work to quickly capture the convention bounce.

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