The accuracy of election polls is under fire. Results from election polls have been part of our daily news diet during the 2016 presidential election. There has been an explosion of institutions, from universities to media outlets, that jumped on the polling wagon in this election year and made it very difficult to decide in which one to believe.
Mentions of results from election polls usually had included sample size and margin of error, but that’s not enough to evaluate their quality. Comparing election polls is nearly impossible without knowing the methodology used in terms of population universe selected, screening criteria, and question formulations. Many of them are flawed, and some are simply not comparable.
Predicting presidential election is risky business, so much so that Gallup, which for many years was considered the country’s gold standard for horse-race election polling decided to give up predictions it failed to predict Obama’s win in 2012.
The 2016 US general election proved to be another defeat for pollsters. The infographic below shows the results from the final poll conducted by Gallup before each Presidential election since 1976. In most years, the polls made a correct prediction of the winner, except for 1976 and 2012.
For 2016, in the absence of Gallup numbers, we used the average across all polls compiled by RealClearPolitics.com during the last few days before 11/8/2016. This method smoothed out the influence of outliers, pegging Hillary Clinton ahead by a few points, although within the margin of error. She won the popular vote, which matched pollsters’ predictions, but lost the electorate vote, becoming the fifth presidential candidate joining this unfortunate club.
It seems the media and pollsters fell victim of the gambler’s bias declaring Hillary Clinton the winner based on many polls pointing in her direction, but most with results within the margin of error. The sheer number of polls with similar results, despite lack of statistically significant differences, blinded everybody. Overlaying directional differences on the electorate map without digging deeper in the nature of the electorate, was the ultimate mistake.