David Rothschild and Sharad Goel write:
In a new paper with Andrew Gelman and Houshmand Shirani-Mehr, we examined 4,221 late-campaign polls — every public poll we could find — for 608 state-level presidential, Senate and governor’s races between 1998 and 2014. Comparing those polls’ results with actual electoral results, we find the historical margin of error is plus or minus six to seven percentage points. . . .
Systematic errors imply that these problems persist, to a lesser extent, in poll averaging, as shown in the above graph.
David and Sharad conclude:
This November, we would not be at all surprised to see Mrs. Clinton or Mr. Trump beat the state-by-state polling averages by about two percentage points. We just don’t know which one would do it.
Yup. 2 percentage points. They wrote this on October 6, 2016.