Polls are likely to move after the debate. It is the moment when voters get to make a direct, side-by-side comparison of the two candidates. This may also be the last time for any significant shift in the race.
Both before and after the debate, pundits will emit opinions about “expectations.” This commentary does not have predictive value. It would be better if they kept their focus on policy substance or factchecking.
Here are three reasons why you should basically ignore the onslaught of horserace punditry that is about to rain down.
It’s a tight race. Clinton’s the favorite but close enough that Trump would probably pull ahead if he “wins” debate. https://t.co/w05RSS9r0M
— Nate Silver (@NateSilver538) September 25, 2016
Above is a mild example of what you can expect in the coming 24 hours. This particular statement is a bit circular: of course Trump will probably be perceived as “winning” if his numbers improve. However, there’s a bigger problem: the premise of “meeting expectations” itself carries no predictive value.
Trump could take the lead, but it would go against what we know so far. I would characterize the race as being very close, but not as uncertain as you might think. Why? The unappreciated story of 2016 is the amazing stability of public opinion. As measured by national polls, 2016 marks the most stable Presidential race in >60 years of modern polling. At the level of state-poll-based analysis, the stability is even greater. This basic fact should inform all analysis.
1. What commentators think about “exceeding expectations” is an anti-indicator. Via Will Jordan and CNN/Gallup, here is survey data on who was expected to do better in past debates.
Notice a pattern: all of these candidates ended up winning the popular vote. The explanation is simple: people can tell who the stronger candidate is, and expect that person to do well. As you can see, Hillary Clinton is narrowly expected to perform better tonight.
Now, think about this for a moment. What does it mean for a candidate to “exceed expectations”? It usually means that he/she was the weaker candidate to begin with. So if some commentator emits hot gas about either candidate doing better than expected, there is a good chance that candidate will be the loser in November.
2. If polls move after the debate, the reasons were baked in a long time ago. Since Phlegm-ghazi broke as a story on September 11th, national surveys have moved toward Trump slightly, then moved back toward Clinton. In polls that included September 12th, the day after Phlegm-gazi, the national margin was Clinton +1.5 ± 0.3% (n=8 polls, median ± estimated SEM). In polls that started on or after September 15th, the margin was Clinton +2.5 ± 0.7% (n=10). This is a pretty small move, but at a minimum it suggests that Clinton’s slight decline in national polls (and the state poll-based analysis that the Princeton Election Consortium provides) has stabilized:
Above, we have a starting point for anything that will happen in the coming weeks: a recent Meta-Margin of around Clinton +1.6%, with a median expectation of Clinton 294 EV, Trump 244 EV.
The overall stability of the 2016 race suggests that compared with today, polls are more likely to move back toward their past average than away from it. Think of public opinion as being like a rubber band: the race has some set point, like a thermostat. If opinion gets far from this natural equilibrium, it gets drawn back to that. This kind of dynamic is often called “regression to the mean.” It is a core assumption of the PEC model.
Given that tendency, I will make a prediction that may sound odd: I have no idea how Clinton and Trump will perform in Monday’s debate. But based on the regression-to-the-mean principle, I expect polls to move toward Clinton in the 1-2 weeks after the debate. Of course, if conditions move in the other direction, come on back and mock me!
3. Polarization has made it difficult for opinion to move much. The volatility in this year’s Presidential race is smaller than any race since the advent of modern polling. This is a symptom of severe political polarization, in which most Trump supporters would never consider supporting any Democrat, including Clinton; and vice versa.
Consequently, opinion is nearly immovable. Whatever happens in the coming weeks, the size of the the change would not have attracted notice in elections preceding 1996.
The groups that may choose up sides are self-described undecideds (4%), Gary Johnson supporters (8%), and Jill Stein supporters (3%). Undecideds and Johnson supporters are likely to split evenly between Clinton and Trump, while Stein supporters should break heavily toward Clinton. Tomorrow is a chance for them to get on the bus.
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