In the US presidential election, the final poll of polls compiled by Real Clear Politics predicted that Hillary Clinton would win 46.8% of the popular vote and Donald Trump 43.6%. In the end, Clinton won 47.7% and her rival won 47.5%. This small majority she had in the popular vote was reversed in the electoral college and she won 228 delegates to Trump’s 279 (figures exclude New Hampshire, Arizona and Michigan). So the last-minute polls were accurate in predicting Clinton’s vote but were off by 4% in the case of the Trump vote. What went wrong?
The failure of the last-minute polls in Britain to accurately call the general election of 2015 and again in the Brexit referendumin June 2016, provides some insights for the US presidential election. The 2015 failure prompted the British Polling Council to set up an inquiry into what went wrong. Four possible explanations outlined in their preliminary report, published in January 2016, of what went wrong in Britain may be relevant to the US presidential election: a late swing, sampling problems, herding behaviour and mis- or “over-reporting”.