Tag Archives: non-response bias

Why Did Polls Predict 2016 U.S. Presidental Election Result So Badly?

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There aren’t many, or any, surveys with more respondents than the polling accompanying U.S. presidential elections. And all such polls have error, both from simple randomness and from non-response bias. This year’s inaccuracy was not the first flagrant example. In 1948 newspapers trumpeted incorrectly that Dewey had beat Truman.

The 2016 election had an important source of randomness and confusion, namely the unpredictable behavior of Donald Trump. And one of the other sources of confusion was the difference between the Electoral College (whose results are the ones used to determine the winner) and the popular vote (which is interesting but not governing). It is still not clear as of November 14 (nearly a week after Election Day), but it appears that Hillary Clinton won the popular vote, and any polling that dealt mainly with the popular vote could easily draw the wrong conclusion.

Additional errors can arise from the inconsistency among the states regarding mail-in ballots or voting places open as early as September 19. The 46 million early voters (prior to election day) included a large number of unaffiliated (neither Democratic nor Republican) voters, making it hard to predict voter behavior on election day. (The total voter count was 130 million.) Beyond this, voter turnout apparently wasn’t predicted accurately

Traditional telephone polling also was a source of errors. Some people intending to vote for Trump were ashamed to admit it when they were surveyed with live interviews. Automated-dialer calls with recorded voice and Internet polling gave better results. But these calls cannot be used with cellphones.

Of all the analyses of the causes of the inaccuracy of the polls, we found the ones by Sean Trende on Rear Clear Politics the most helpful. Most perceptive was his finding that the polls were OK, but the conclusions by the pundits weren’t.

Presidential Race Polls Inaccuracy Is Exceeded Only By Their Number

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Who needs the comic strips in the morning papers these days? The latest antics of the U.S. presidential candidates and the pollsters are a lot more amusing. And this phenomenon is not new; we even featured it in our December 2014 post regarding the off-base headline trumpeting the victory of Dewey over Truman (sic). In alll fairness, it is not at all easy to do a good political poll, and the current campaigns in the U.S. may be among the most weird in history, due to the unpredictable behavior of Donald Trump, the political baggage of Hilary Clinton, and most recently Ted Cruz’s claim that Ben Carson was dropping out of the race. But there are some big limitations in the polls themselves, most importantly that so many people polled don’t vote and so many people who do vote don’t get polled. And these gaffes are not limited to the U.S. In May 2015 in the U.K. the polls predicted that the vote would be divided equally between Conservative and Labour candidates, but the actual results favored the Conservatives, and Prime Minister David Cameron. Again, the villain was a poor choice of the sample, which included too many younger and politically active voters at the expense of under-sampling older voters.