Why The Polls Missed Bernie Sanders’s Michigan Upset
Here is our initial assessment of some possible explanations, along with comments from some of the pollsters who had reported a big Clinton lead:

Pollsters underestimated youth turnout. Voters under 30 made up 19 percent of Democratic primary voters, nearly as large a share as voters 65 or older, according to exit polls. Mitchell Research and Communications, which showed a 37 percentage point Clinton lead in a poll conducted Sunday, found that people younger than 50 would make up less than a quarter of all voters; they made up more than half instead. Mitchell was one of the only pollsters in the state to poll using only calls to landlines, and most Americans younger than 45 live in households without landlines. But even Monmouth, which dialed cellphones, too, underestimated the turnout among younger voters. Perhaps all the polls showing a big Clinton lead sowed complacency among Clinton supporters, who skew older — though big leads in polls in Southern states didn’t stop her supporters from helping her romp to big victories.

Pollsters underestimated Sanders’s dominance among young voters. Not only did more young voters turn out than expected, but Sanders won 81 percent of 18- to 29-year-olds. A YouGov poll showed him winning 66 percent.1

Pollsters underestimated the number of independent voters who would participate in the primary. YouGov expected Sanders to beat Clinton by 38 percentage points among independent voters participating in the open Democratic primary. He won those voters by 43 percentage points. But no one expected independents to make up 27 percent of voters; YouGov expected about 12 percent. “There were too many Democrats in the poll,” said Will Jordan, elections editor at YouGov.