The most striking feature of polling in 2016 was not that a few polls had large errors. That always happens. Rather, it was the error in the polling averages. If all the polls overestimate support for one candidate (as most of the 2016 polls did for Clinton), then there will be no negative errors to cancel out the positive errors and averaging fails. After 2012, some had mistakenly assumed that polling averages didn’t have any error. This was a misreading of 2012 (35 of the last 36 polls in 2012 underestimated Obama’s lead), but hardly anyone noticed, since we got the outcome of the election right.
Another strange feature of this year’s polling that three weeks before the election there was wide disagreement among the polls. A few polls had Clinton with double-digit leads, while others showed it to be a close election. A few, like ours, showed very little movement during this period. It is unclear why the polls converged. It could be due to bad behavior on the part of some pollsters (“herding”) or, more likely, the use of similar methods that had some defect pushing the polls in Clinton’s direction. In particular, most polls underestimated Trump support in the midwest, where he did better than expected with white working class voters. This too deserves further investigation.
At this point, we don’t know what happened in the places where our polls were off. We have lots of hypotheses, but instead of speculating, we’re collecting more data. Every person that we interviewed before the election is being reinterviewed after the election, to see if there was much switching or last minute deciders. As state voter files become available, we will check on who voted and when and how they voted. Stay tuned.
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