It's been called one of "the greatest polling errors in primary history" and it happened right here in Michigan. Just 8 months ago the experts predicted that Hillary Clinton would defeat Bernie Sanders by 21 points in the Democratic primary. Not only did Clinton not win by 21 -- she ended up losing.

So what went wrong? We asked Dr. John Clark of Western Michigan University -- an expert on political polling.

"There are different ways to contact potential voters to try and figure out how they're likely to vote, the most common that we see during election season are different kinds of telephone survey, some of them have live interviews, some of them use robo-calls and some of them are focused exclusively on people that have land-lines," says Dr. Clark.

Sanders brought out young voters in Michigan -- many of whom don't even have a landline. Now the polls have Clinton beating Trump by 7 points in Michigan. So can we trust these numbers?

"My answer is sure you can trust them, but do you understand them? If you understand that even under the best of circumstances, they might not be spot on but they are probably accurate within a range, then sure you can trust them. If you understand that there are some people, some people that live in my own home that are never going to answer a phone call that comes from a pollster and there are other people who always going to answer a phone call from a pollster, if you understand that, then sure you can trust them," says Clark.