Why The Polls Missed Bernie Sanders’s Michigan Upset

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If Bernie Sanders were to defeat Hillary Clinton in Michigan’s Democratic primary, it would be “among the greatest polling errors in primary history,” our editor in chief, Nate Silver, wrote Tuesday evening when results started to come in. Sanders pulled it off, and now we’re left wondering how it happened. How did Sanders win by 1.5 percentage points when our polling average showed Clinton ahead by 21 points and our forecasts showed that Sanders had less than a 1 percent chance of winning?

With a polling miss this big, no single factor is likely to explain it, so more than one answer could be correct. Also, not every pollster releases detailed data, and it may take some time to fully diagnose what went wrong. “It’s a little bit of everything,” Monmouth University pollster Patrick Murray told The Huffington Post.

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, andmost 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
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