In the Marquette Law School Poll released yesterday, the big take away wasn't that Mary Burke and Governor Scott Walker are tied among registered voters, it was the shocking revelation that for the first time in the Marquette poll's history, there was a Republican advantage in the sample of registered voters.
What does that mean? Every poll is weighted in an attempt to accurately reflect the demographics of the total population. In Mississippi, for example, we know that there are many more Republicans than Democrats and we have a good idea what the proportions should be based on previous elections. A poll in Mississippi, therefore, would be weighted with more Republican participants. Of course, some voters change their minds, during the course of an election cycle, as to which political persuasion they lean towards. So the challenge for a pollster is to take the evidence of who showed-up in previous elections and balance that with what the sample of any given poll is telling them and then try to come up with an appropriately weighted sample.