The economy was certainly the largest factor in Michigan’s GOP primary last night, as an MSNBC exit poll showed. Fifty-five percent cited that as their top concern, 18 percent said Iraq and 14 percent said illegal immigration were the dominating issues.

Romney’s upbeat economic message helped propel him to victory. But that wasn’t necessarily predicted by the polls leading up to the primary, which were wildly different. Of those taken over the last week, one had McCain winning with 34 percent of the vote (Jan.9-11 survey), another showed an almost dead heat, McCain with 27 percent, Romney with 26 percent. The one thing they shared in common was a large number of undecided voters – as high as 16 percent. Of the six polls we wrote about in Poll Tracker, there was a 50-50 split with Romney and McCain each topping three of them.

Who came the closest? The Mitchell Interactive Poll with Romney at 35 percent, McCain 29, Huckabee 12. The actual results were: Romney 39 percent, McCain 30 percent and Huckabee 16 percent.

Why so different? In the statistics world, it’s called confounding variables, which make it hard to compare polls. Some of the polls were done with likely Republican voters only, others included independents and Democrats, who did not turn out as much as some pundits suggested. Some polls were conducted Thursday-Sunday, others Friday-Monday. Some had larger margins of error than others. And, there was the weather factor; cold and snow may have discouraged some voters. And the large number of uncommitted voters could have been the most confounding of all.

Of course all polls, no matter how well they’re done, have a chance of being wrong – it’s just the random variation of sampling.