Andrew Gelman notes that if you look at historical polls and check the total error vs just the sampling error, the latter is about half the size (implying that systematic error is as large as sampling error, but unfortunately much harder to deal with as it is unsystematic over time), and the 2016 election margin fell within the confidence interval you would expect from that: http://www.nytimes.com/2016/10/06/upshot/when-you-hear-the-m... http://researchdmr.com/ProbabilityTotalError.pdf http://andrewgelman.com/2016/11/11/david-rothschild-sharad-g...

And statisticians have pointed out the inherent limits to surveys/polls in election forecasting since at least the Literary Digest debacle; a particularly good discussion is in Schlaifer 1959 https://www.gwern.net/docs/statistics/1959-schlaifer-probabi... (excerpt: https://www.reddit.com/r/slatestarcodex/comments/5c10jx/post... )