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Sunday, September 30, 2012

Forecasting the Presidential Election

At The Washington Post, Dan Balz writes:
Are you ready to call the election? Mitt Romney certainly isn’t, nor for that matter is President Obama. But a few hardy academics have done so. Out now are a baker’s dozen forecasts produced by political scientists that predict the outcome in November.
Polls give Obama the advantage, nationally and in most of the battleground states, but they are, as is often said, snapshots in time, not predictions of the future. The election forecasts are in fact predictions, based on various and varied statistical models. Most give the advantage to the president, but the verdict is not unanimous.

The 13 projections are contained in the new issue of PS: Political Science and Politics, which is published by the American Political Science Association. Eight of them project that Obama will win the popular vote; five say the popular vote will go to Romney. But the degree of certainty in those forecasts differs. One projection favoring the president says there is an 88 percent certainty that he’ll win, while two others forecasting Obama say there is only a 57 percent certainty.
James E. Campbell, the department chairman at the University at Buffalo in New York, who wrote the introduction to the package, rates them this way: Five predict that Obama will win a plurality of the two-party vote, although three are on “the cusp of a toss-up.” Five predict that Romney will win the plurality of the two-party vote. Three are in what he calls the toss-up range.
Campbell offers some wise observations about forecasts in general:
The rationale behind many of the statistical election forecasting models is (1) that we can identify general influences that normally influence the vote (the fundamentals, such as the economy and incumbency), (2) that many of the fundamentals are known and measured before the general election campaign begins, (3) that these fundamentals shape how normal campaigns are likely to affect the vote, and (4) that their typical effects on the vote can be estimated based on the history of past elections. Note that forecasting does not necessarily assume the lack of campaign effects on the vote or that campaigns do not matter. Campaign effects themselves may be shaped by the fundamentals (Campbell 2008). Some models also start from a precampaign public opinion baseline and assess those influences that may affect the development or change in voter preferences during the campaign. Whether using a precampaign public opinion baseline as a starting point or not, this line of reasoning is the foundation of the enterprise. It would not seem to be terribly controversial or particularly mysterious.
Of course, the devil is in the details. The specific factors included as fundamentals, their appropriate indicators, the time over which their typical effects are estimated, the lead time before the election for the forecast, and other choices create a diverse array of models and forecasts. This diversity is often overlooked. A substantial difference (often lost on critics) exists between evaluating election forecasting models as an enterprise and evaluating individual models. Some models are considered more credible than others. Some have longer and stronger track records than others. Some are more consistent with the existing body of voting behavior and campaign effects research. By the same token, there is an important distinction between evaluating each model and its forecast for a particular election. Each election prediction offers a real test of a forecasting model, but as important as it is, it is only a single test.
James E. Campbell, "Forecasting the 2012 American National Elections," PS: Political Science & Politics, Volume 45, Issue04, October 2012 pp 610-613,