538 Model Megathread

[Mr. Morden]

Serious question - why does the Nowcast do a trendline? That doesn't make sense to me. If Clinton leads in Nevada now (as she does per their poll model) why would the Nowcast not predict her as ahead?

Maybe I'm misunderstanding something, but the latest 5 it's showing have Trump up in 3 and Clinton up in 1. The most recent is Clinton +4, but it gets adjusted to a tie based on other factors (e.g. national polling, covariance with other states). So I guess it doesn't surprise me.

If you look at the Nevada Nowcast it has a polling average of Clinton +1.5%.

Adjustment for some of their stuff brings it to Clinton +1.2%.

THEN they adjust for "trend" which swings it by 4.5% in Trump's favour and gives it as Trump +3.4%.

I don't get what trend they're adjusting for if it's "election held today". The net swing is roughly the same as in the forecasts for November 8th.

They are looking at how the national polls have changed since those Nevada state polls were conducted, and assuming Nevada has gone to the right at the same pace since then as the rest of the country (even though we don't have recent Nevada polls to confirm this).

Yeah, now-cast doesn't mean you take all the polls as is at the time they were taken.  It means you take the polls and adjust them for any trends in national polls that have happened since they were taken, to see what they would look like if taken right now.

I thought about that but then why are the trends not a lot stronger for the November forecasts?

If a particular poll in a given state was taken a month ago, and Trump has gained nationally since then, then you adjust the poll in a Trump-ward direction to figure out what the margin would be if the poll was taken now.  But there's no reason to assume that same trend keeps going in the same direction between now and November.  We have no idea what the trend is going to be between now and November.

So I'm not sure on the details of how the now-cast number differs from the polls-only number.  I guess there's some difference between the predictive power of a poll taken immediately before an election and one taken months beforehand, and the model reflects that, but I don't know the guts of it.  I'm assuming that one facet of it would be that now-cast places heavy weight on polls taken within the past week as opposed to polls taken a month ago, because if the election was held today, then polls taken within the past few days would be highly predictive.  However, if the election is three months away, then the weighting on more recent polls isn't as heavy.  Polls taken three months before an election might not be hugely more predictive than polls taken four months before an election.

[Erich Maria Remarque]

I am surprised the Nov Polls-only forecast has Trump's chances at 47%.   I would imagine the model takes into account the temporary GOP post-convention bounce.   

It did

It doesn't. Only polls-plus does that.

It does, but much less aggressively than the poll-plus model. The poll-only model gives latest polls less weight even though they... yeah, recent.
http://projects.fivethirtyeight.com/2016-election-forecast/national-polls/

[Illiniwek]

Jesus are you kidding me Nate? I get that trump got a bump but this is ridiculous.

[Nym90]

As I understand it, the difference between now-cast and polls-only is that polls-only assumes some regression to the mean between now and November, thus she is still ahead because it considers the polls more likely to move in Clinton's direction between now and then than it does Trump's. It also values higher quality, older polls (which were more favorable to Clinton) over lower quality, newer polls that favor Trump (you may notice that the more highly rated pollsters, wisely IMO, are avoiding polling between conventions).

Now-cast puts more of a premium on recent polls than on polls with a better track record, and assumes the election is today and thus no further movement can occur.

[Pragmatic Conservative]

So New Jersy is now a potential tipping point state or is that an error?

[Gass3268]

So New Jersy is now a potential tipping point state or is that an error?

2-3% chance as a tipping point state.

[The Other Castro]

The Now-Cast now shows a 270-268 Trump win if you assign all state to their most likely outcomes, with ME-2 giving Trump the win.

[Gass3268]

Hillary is essentially in the same place in the polls-plus model right now as she was on June 8th when the model started.

[5280]

I'm surprised Trump could actually win Nevada and lose Colorado at this point when looking at polls.

[Gass3268]

I'm surprised Trump could actually win Nevada and lose Colorado at this point when looking at polls.

He could, but based on past elections I would say that Trump needs to be up on average by 5% on election day to have a chance.

[Likely Voter]

I continue to be skeptical of the IA, NH, NV, ME2 bank shot, but that is where the polls say is Trump's best shot.  The biggest question would be, are the NV polls right this time and has it really moved to be more R than VA, CO and PA?  Also is NH really more R than NC? Really? probably not.

There is a reason why Nate Silver's book was titled "The Signal and the Noise"

[Gustaf]

I think they forgot to update their nowcast. Pennsylvania should be flipped.

[Erich Maria Remarque]

For the first time Trump leads in polls-only forecast 50.1% vs 49.9%. Of course, it's due RNC-bounce, since we don't have any post-DNC polls yet. But still...

[Wells]

For the first time Trump leads in polls-only forecast 50.1% vs 49.9%. Of course, it's due RNC-bounce, since we don't have any post-DNC polls yet. But still...

That's weird because the now-cast has gone back to a Clinton victory.

[Angel of Death]

Trump doing better in the polls-only forecast than the now-cast would suggest a DNC bounce, as the latter is more aggressive in response to new polls.

[Deblano]

Trump doing better in the polls-only forecast than the now-cast would suggest a DNC bounce, as the latter is more aggressive in response to new polls.

I'd say that we have to wait a few more days (Wednesday perhaps?) until we can truly find out whether there has been some sort of a post-DNC bounce for Clinton.

[100% pro-life no matter what]

50.1%-49.9%

[Lief 🗽]

Sad that Nate Silver, who is totally discredited and a fraud, is now whoring himself out for page clicks like this. Not good!

[hopper]

Polls-Plus showed Trump not getting any convention bounce for Election Day 2016. His odds still stayed at 40% of winning the election. Maybe I should go back to polls-plus instead of now-cast.

[Xing]

I'm still trying to wrap my head around why they have Minnesota so ridiculously close. I mean, I get it, Mason-Dixon isn't the best pollster, but they're only counting two polls from Mason-Dixon, and then massively unskewing them. Do they really think Minnesota will be a two-point race, even if Trump narrowly wins?

[Erich Maria Remarque]

Polls-Plus showed Trump not getting any convention bounce for Election Day 2016. His odds still stayed at 40% of winning the election. Maybe I should go back to polls-plus instead of now-cast.

The polls-plus model assumes, that there is 4% convention bounce for each candidate, and that is about what Trump got on average.

[Erich Maria Remarque]

I'm still trying to wrap my head around why they have Minnesota so ridiculously close. I mean, I get it, Mason-Dixon isn't the best pollster, but they're only counting two polls from Mason-Dixon, and then massively unskewing them. Do they really think Minnesota will be a two-point race, even if Trump narrowly wins?

But they write why, don't they? It mostly depends on trend line and house effects.


1.   Polling average   46.7%-   35.8%-   — -
Adjust for likely voters   +0.6--   +0.7--   —--
Adjust for omitted third parties   -3.6--   -3.6--   —--
Adjust for trend line   -3.3--   +3.2--   —--
Adjust for house effects   -1.4--   +0.8--   —--
2.   Adjusted polling average   39.1%-   36.9%-   — -
Allocate undecided and third-party voters   +7.2--   +7.2--   —--
3.   Polls-based vote share   46.3%-   44.1%-   — -
Calculate demographic regression   45.7%-   44.7%-   8.4%-
4.   Projected vote share for Nov. 8
Weighted average 52% polls-based, 48% demographics
46.0%-   44.3%-   8.4%-

[Wells]

The current Now-Cast map:

Clinton 342
Trump 196

[heatcharger]

We need new NC polls badly. Trump being favored in the polls-only and polls-plus forecasts makes no sense considering all the recent polls have Hillary ahead.

[windjammer]

The current Now-Cast map:

Clinton 342
Trump 196

RIP TN Volunteer