In 2016, state polls understated Republican support by large margins in many small states. In the five states in bold, Hillary Clinton lost the statewide vote despite leading in the polls. Key Pre-election

polling average Election

result

It was the biggest polling miss in a presidential election in decades.

Yet in many ways, it wasn't wholly out of the ordinary.

Over all, the national polls missed the result by only a few points: Hillary Clinton is on track to win the popular vote by around 1.5 percentage points, not especially far from her roughly four-point lead in an average of national polls.

But the state polls were a different story. They systematically underestimated Donald J. Trump's standing in the Upper Midwest and Northeast. His strength there was enough to make him the president.

Few saw it coming. Mrs. Clinton led in nearly every high-quality survey of Minnesota, Maine, Pennsylvania, Wisconsin and Michigan. It was her modest but consistent edge in those states and others that led analysts to project that she had an excellent chance at victory. This includes our poll-based forecast, which gave Mrs. Clinton an 85 percent chance to win, alongside those from other organizations, which put Mrs. Clinton's chances at between 71 percent at FiveThirtyEight and greater than 99 percent at the Princeton Election Consortium.

Her campaign, with its own polling, concluded the same thing. She barely even campaigned in Maine, Wisconsin or Minnesota, and visited Michigan only in the last week of the race.

Yet in the end, her polling lead proved illusory.

All of these states have something in common: They have a large number of white voters without a college degree. Mr. Trump also outperformed the polls in other mostly white and rural states, whether Alaska, Iowa, Kansas, Missouri or Indiana.

The states with a large number of white working-class voters tend to be somewhat less populous than the more diverse and well-educated states along the coasts. A result is that the state polling averages were off by more than usual, even though the national numbers weren’t far off.

State Polling Errors in 2016 Were the Largest in Decades Average absolute difference between polling average and final vote in the ten states closest to the national average with at least three polls. 1988 1992 1996 2000 2004 2008 2012 2016 3.4 pts. 3.4 pts. 2.3 pts. 1.8 pts. 1.7 pts. 1.7 pts. 2.3 pts. 3.9 pts. Average absolute difference between polling average and final vote in the ten states closest to the national average with at least three polls. 1988 1992 1996 2000 2004 2008 2012 2016 3.4 pts. 3.4 pts. 2.3 pts. 1.8 pts. 1.7 pts. 1.7 pts. 2.3 pts. 3.9 pts.

Mrs. Clinton tended to outperform in big, liberal states with larger Hispanic populations. That was true in battlegrounds, like New Mexico and Nevada. It was also true outside the battlegrounds, in Illinois, California, New York and Washington.

Over all, the two types of misses nearly canceled out in national polls. But Mr. Trump's gains among white working-class voters were far more important, because those voters are overrepresented in the most important battleground states.

Analysts are only beginning to unpack how the polls were so off in the Midwest and Northeast. It will be hard to know until voter file data — which includes information on exactly who voted — is updated. That could take months.

But there are already theories. The polls’ samples might have overrepresented well educated voters. The educational composition of the electorate is one of the biggest mysteries for pollsters. It’s not included in the voter file and it’s hard to model (it’s much harder to tell how educated people are based on their names or where they live; race is easy by comparison).

Or undecided, well-educated Republican voters in the suburbs around Milwaukee, Minneapolis and Grand Rapids might have broken for Mr. Trump at a higher rate than expected. The polls may have slightly overestimated black turnout as well.

Was this polling miss unusual? The size and direction of the error in 2016 was big, but not totally out of the norm. This was part of the reason we had been writing about the possibility of Mr. Trump’s victory and cautioning that an 85 percent chance was not a 100 percent chance. (In comparing Mrs. Clinton’s chances of losing to the chances that an N.F.L. kicker would miss a 37-yard field goal, we tried to give an example of a low-probability event that nevertheless still happens. See also: The Cubs rallied to win the World Series.)

Should we have given Mr. Trump higher odds of winning? It’s possible. The history of polling errors did not suggest so, but that history now has one more election with several consequential misses.

In 2012, the state and national polls were off by a similar amount, but in the other direction.

2012

The polls systematically underestimated President Obama's support across the battleground states, and nationwide. But it didn’t really matter that he won many states by a few more points than state polls suggested. If the polls had been off in the other direction by the same amount, Mitt Romney would have squeaked out a win.

Below, the state polling errors in the presidential elections since 1988:

2008

2004

2000

1996

1992