Today's election of Donald Trump in the United States will be added to the list of historic polling fails.
- The New York Times had the average of the polls with Clinton on 45.9 per cent to Trump's 42.8 per cent (+3.1 percentage points). The NYT gave Clinton an 84 per cent chance of winning the Electoral College vote.
- FiveThirtyEight.com had the average of the polls with Clinton on 48.5 to Trump's 44.9 per cent (+3.6). FiveThirtyEight gave Clinton a 71.4 per cent chance of winning the Electoral College vote.
- The Princeton Election Consortium had Clinton ahead of Trump with +4.0 ± 0.6 percentage points. PEC gave Clinton a 93 per cent chance of winning the Electoral College vote.
While the count is not over, the current tally has Clinton ahead in the national popular vote by +1.1 percentage points, but losing the Electoral College vote. The most likely Electoral College tally looks like Trump with 305 Electoral College votes to Clinton's 233.
So today's big question: Why such a massive polling fail?
It will take some time to answer this question with certainty. However, I have a couple of guesses.
My first guess would be the social desirability bias. This is sometimes referred to as the "shy voter problem" or the Bradley effect. At the core of this polling problem, some voters will not admit their actual polling preference to the pollster because they fear the pollster will negatively judge that preference. It is not surprising that such a controversial figure as Donald Trump would prompt issues of social desirability in polling. Elite opinion was against Trump. Clinton labeled Trump supporters as "deplorable". No-one wants to be in that basket. Pollsters might also look at Latino voters in Florida that appear to have voted for Trump in larger numbers than expected.
The second area where I suspect pollsters will look is their voter turn-out models. Who actually voted compared with who said they would vote to pollsters. This was a very different election to the previous two Presidential elections. Turnout-out models based on previous elections may have misdirected the polling results (particularly on the basis of race and particularly in the industrial mid-west).
A final thing that might be worth looking at is herding. The final polls were close, perhaps remarkably close. This may have been natural, or it may have resulted from pollsters modulating their final outputs to be similar with each other.