In today’s New York Times Nate Silver establishes President Obama as a prohibitive favorite in Tuesday’s electoral vote contest. His projection is based largely on the information in state polls in key swing states. While national polls suggest that the popular vote total will be close, Silver’s simulations indicate that President Obama has an 85% chance of winning re-election on Tuesday. Silver’s projections, which have come under attack from Romney supporters in recent weeks, rely on a combination of state, national and tracking polls. His state-by-state forecasts use a sophisticated algorithm to weight information based on the accuracy of previous polls, the number of undecided voters in each state, and recent trends.
I have no doubt that the President is the favorite to win re-election on Tuesday because Mitt Romney needs to win the vast majority of swing states in order to reach 270 electoral votes. Silver, a successful former poker player, has established Mitt Romney as an 11:2 longshot. Given the closeness of the national polls it may seem tempting to some to question Silver’s forecast. Could Silver possibly be wrong? The answer rests with his reliance on state polls, and the accuracy of state polls in swing states where the races are fairly close.
In today’s column Silver makes a convincing case that state polls are accurate, on average, especially as we get close to election day. He states:
“Of the 77 states with at least three late polls [since 1988], the winner was called correctly in 74 cases …There has been little tendency for the state polling averages to overrate either Democrats or Republicans, or either incumbents or challengers.”
I examine only the 60 state poll averages since 2000 listed by Silver (and ignore the small group of state polls he lists from 1988-1996). Silver is correct that state polls are correct, on average. The average difference between the election outcome and the late poll in a state is about one-quarter of one percentage point. Nonetheless, state polls tend to lean one way in one election and another way in a different election. In 2000, Al Gore outperformed state poll projections by an average of 2.14 percentage points. In 2004 George W. Bush outperformed state poll projections by an average of 1.52 percentage points, a swing of 3.66 percentage points in just 4 years. In addition, John McCain outperformed state polls by 0.70 percentage points in 2008. If the prediction error in state polls was truly independent of all factors, we would only expect to see such large differences in average prediction errors across elections less than one time in three hundred (less than once every millennium).
What does it mean that state polls lean in different directions from election-to-election? As a practical matter it means that once we observe the difference between actual election outcomes and late state polls in a state such as New Hampshire on Tuesday, we will have a better idea what will actually happen in Florida, Ohio, Virginia and other swing states. In the language of statistics the forecast errors from polls are correlated among states in the same election cycle. These correlations within the same election cycle mean that the candidate that exceeds expectations in one state is likely to exceed expectations in other states as well.
State polls are clearly missing something that varies from election-to-election. I have no idea whether the late state polls in 2012 will miss the forecast because of turnout, momentum, the political preferences of late deciders or how voters who refuse to answer pollsters will actually cast their ballots. I am also not sure whether the forecast errors will favor the President or Mitt Romney. What does seem plausible, however, is that the difference between late state polls and actual vote totals will be more than one-quarter of a percentage point, the overall average since 2000.
When Silver establishes the President as an overwhelming favorite it is based on a model that has been estimated over a relatively small number of situations in recent elections where the race was expected to be close at the state level. The projected margin of victory/loss in state poll averages was within 6 percentage points only 28 times in the three presidential elections from 2000 to 2008. To statisticians, that is a fairly small sample. But more importantly, when one state poll tends to underestimate the strength of a candidate, whether Democrat or Republican, other state polls tend to make the same mistake. Silver’s forecast is betting that state polls will be very accurate in 2012, which is different from saying they have been accurate, on average, across several elections. So if you see Nate Silver and he offers you 11:2 odds on a bet that Mitt Romney wins the election, you might want to take it. But be careful of his poker face.