How accurate are polls at predicting a winner?
Not too. So long as a candidate is within 10 points, most polls shouldn’t be readily relied on as predictors for who will win. Charles Franklin, a political science professor at the University of Wisconsin has an interesting post today about just how important the “margin of error” really is. On a graph, Franklin compares poll results with actual election results, resulting in several observations, one of which is the importance of realizing that polls cannot reliably predict races that are less than 10 points apart. One interesting feature is that a margin of zero (a tied poll) produces a 50-50 split in wins with remarkable accuracy. There is nothing I did statistically to force the black trend line to go through the “crosshairs” at the (0, .5) point in the graph, but it comes awfully close. So a tied poll really does predict a coin-flip outcome. The probability of a win rises or falls rapidly as the polls move away from a margin of zero. By the time we see a 10 point lead in the poll
