When voters in the United Kingdom went to the polls on June 23, 2016 to vote on the referendum on whether to leave the European Union (Brexit) or remain in it (remain), the polls all predicted – by an average of 8% – that “remain” would win handily. But of course we all know that the actual vote ended with a 52-48% Brexit victory. Five months later, all the polls predicted Hillary Clinton would defeat Donald Trump in the U.S. presidential contest, with the New York Times at one point giving Clinton a 99% chance of prevailing. In the end,...