Polling companies have most definitely come under fire in recent political history. The closeness of the polls, at one period, prior to the Scottish Independence Referendum of 2014, sent many into a tail spin. The outcome ended up being circa 55%-45% in favor of remaining in the UK. Brexit was treated as a “shock”, as was the election of Trump, and the recent British General Election has put the polling companies under scrutiny again (as did the 2015 election, which saw David Cameron produce a surprise majority for the Conservative party, paving the way for an EU referendum).
The American Association for Public Opinion Research has published a report that analyzes the performance of the polls during 2016. Why was Trump underestimated? Did many voters change their mind just before polling day? The American Enterprise Institute for Public Policy Research has produced an article that looks at some of the key points from the data released in the aforementioned report.