Today I present a beta version of the sharpened forecast. In May, I said that I would update the model after the dynamics of this year’s race became clear. Back then, I wondered whether the 2016 campaign would be more like 1952-1992 (high variability), or like 1996-2012 (low variability). This year’s data indicate that it is the latter – opinion is relatively stable. That stability affects the November forecast.
This is an open comment period. Technical feedback is welcome. (The comments section is rather good this time.)
The Princeton Election Consortium’s prediction of November outcomes is based on the assumption that the race, as measured by the Meta-Margin, can drift in either direction between now and Election Day. The Meta-Margin, which is calculated entirely from state polls, is measured in units of margin between the two major candidates. Today it is Clinton +5.5%, which means that Hillary Clinton would defeat Donald Trump in the Electoral College, and polling margins would have to move across the board by 5.5% toward Trump in order to create a toss-up. If the change is larger, then Trump would be favored to win.
To understand how likely such a change would be, we have to have a measure of volatility during the general election season. That measure is the standard deviation. Since 1952, it has looked like this:
From 1952 to 2012, the two-candidate polling margin has had a typical standard deviation of 6% during the general election campaign. In other words, if you compare any two moments in time during the campaign, national polling margins at those two moments will usually be within 6% of each other – about two-thirds of the time, to be precise. If the two moments are close in time – for instance October 1 and Election Eve – the difference would be even smaller. This allows us to make a prediction about the future.
However, something changed starting in 1996. As I have written before, national politics in the United States became dramatically more polarized starting around 1994, when Newt Gingrich led Republicans to take over the House and Senate. Since then, as the graph shows, Presidential campaign dynamics have gotten much more stable. National polling margins have varied by only 3% on average. The Meta-Margin is even more steady, with a standard deviation of 1-3%.
The same point is obvious when examining the original time series plots of Wlezien and Erikson, whose data I used to make the graph above.
It is now apparent that 2016 is more like 1996-2012 than it is like 1952-1992. Data points for 2016 are included in the graph above. Even though the breakdown of the Republican Party and the advent of Donald Trump have made 2016 a crazy political year, public opinion is more stable than ever.
The Princeton Election Consortium’s initial assumption was that 2016 would be as volatile as typical campaigns since 1952 (SD=6%). This was a conservative assumption with lots of uncertainty. It was consistent with claims by pundits – and by Republican candidate Donald Trump – that the electoral map was scrambled. However, that scrambling has not materialized. Obama blue states are still Clinton blue states, and with a few exceptions, Romney red states are still Trump red states. This again shows that voters are highly entrenched in their views.
And here is what a “low variability” assumption (SD=3%) gives:
Starting today, we will use a lower-variability assumption. This makes the November win probabilities higher for the leading candidate, Hillary Clinton. The random-drift win probability goes from 78% to 92%, and the Bayesian win probability goes from 86% to 95%.
Finally, there is another step, which is converting the somewhat obscure Meta-Margin back into electoral votes, which is far better known. This has also been updated. Previously, the conversion was done using June polling data and 2012 election results, which led to a hard ceiling for Hillary Clinton because Romney’s wins were fairly large. Now that we have polls in more states, it has emerged that both Clinton and Trump have some fairly weak leads. That means that a small swing of opinion in either direction now leads to corresponding changes in EV. The “electoral strike zone” now reflects that possibility.
Going forward to Election Day, the electoral strike zone will be based on the latest polling data.
The change not only makes a Trump win less likely. It also makes an electoral blowout by Clinton less likely. It might also make a switch in House control less likely.
By the way, originally I was going to wait until after Labor Day to make this modification. It would have been a logical time. However, the variability in national poll margin and in Meta-Margin is not going to change in the coming two weeks. Plus I am impatient.
Also…Labor Day is in only two weeks. Summer is nearly over, everyone!
Update, August 22, 3:00pm: The best counterargument in the comments section is the issue of undecided/third-party voters. This year, that fraction is at 20.5% of voters; in 2012, the corresponding number was 9.5%. This raises the issue of whether this year is going to be like 1968/1980/1992/1996, years where the third-party candidate got 7-19% of the popular vote. In those years, the SD ranged from 4% to 14%.
Graph credits: Thanks to Prof. Christopher Wlezien of the University of Texas for the national polling data, which forms a major foundation for his book with Robert Erikson, The Timeline of Presidential Elections. The Meta-Margin data come from past calculations here at the Princeton Election Consortium.