We wrote before the election that a polling error of 2 to 3 percentage points is normal these days... [L]et’s step back and look at the different kinds of polling error. All of them are important because all of them were present in this result.
Every poll has error, some from statistical noise and some from factors more difficult to quantify, such as nonresponse bias.
Poll-based forecasts such as ours attempt to reduce error by combining many different polls and accounting for their quality and lean... It’s possible for polls to be wrong in many states but not in the same direction. These errors could then cancel each other out — or not matter at all, if they’re smaller than a candidate’s winning margin.
But, more often, state polls and the forecasts based on them miss in the same direction. That’s a more systematic polling error, indicating that pollsters were struggling with the same challenges no matter where they were polling or their particular methodology. That also shows up in the plentiful national polls, which we use to adjust our state polls.
Errors of all of those types added up to Tuesday’s result. Individual polls were wrong. Aggregated, they missed in individual states, including in many swing states. National polls were off in the same direction: Polls overstated Clinton’s lead over Trump...
While the errors were nationwide, they were spread unevenly. The more whites without college degrees were in a state, the more Trump outperformed his FiveThirtyEight polls-only adjusted polling average, suggesting the polls underestimated his support with that group. And the bigger the lead we forecast for Trump, the more he outperformed his polls. In the average state won by Trump, the polls missed by an average of 7.4 percentage points (in either direction); in Clinton states, they missed by an average of 3.7 points. It’s typical for polls to miss in states that aren’t close, though. The most important concentration of polling errors was regional: Polls understated Trump’s margin by 4 points or more in a group of Midwestern states that he was expected to mostly lose but mostly won: Iowa, Ohio, Pennsylvania, Michigan, Wisconsin and Minnesota.