He's Up a Point! No, He's Down!

Approximately 250 million adults live in the United States. 1% of that total equals 2.5 million people. Bear that in mind every time you hear somebody say that a poll shows Romney or Obama up a point or down a point from yesterday or the day before. Did 2.5 million people really just change their minds? Or are we experiencing simply the inexactitude of our instruments, statistical noise which crackles sometimes a little louder, sometimes a little softer?