In this year's race, national polls show a tie, while state polls show a decisive Obama advantage. The discrepancy has interested many commentators on the right: Karl Rove, Ross Douthat. Also Andrew Sullivan. Today at the Princeton Election Consortium, I offer a resolution to the apparent problem. I suggest that the difference may arise from the fact that the same systematic pollster errors can have different effects depending on whether they occur in national vs. state surveys.
Based on past elections, national poll aggregates differ from election results by as much as 2.5%. During the same period, state-poll aggregation has been considerably more accurate. Even if state polls have the same accuracy as national polls, races at that level are usually decided by larger margins, leaving room for aggregation to remove the effect of the error. I suggest that the Meta-Analysis of state polls provides a more accurate poll-based prediction of next Tuesday's outcome than national polls.