A 2016 Review: Why Key State Polls Were Wrong About Trump

Nearly seven months after the presidential election, pollsters are still trying to answer a question that has rattled trust in their profession: Why did pre-election polls show Hillary Clinton leading Donald J. Trump in the battleground states that decided the presidency? Is political polling fundamentally broken? Or were the errors understandable and correctable?

At their annual conference in New Orleans this month, polling experts were inching toward the latter, more optimistic explanation. And there is mounting evidence to support their view.

At least three key types of error have emerged as likely contributors to the pro-Clinton bias in pre-election surveys. CONT.

Nate Cohn, New York Times