Thursday, October 2, 2008

Election Forecasts

A new symposium on forecasting the 2008 presidential election is now available online in PS:Political Science and Politics. This is a collection of forecasts from several political scientists, including me. The crucial table (below, click on it to make it easier to read) comes from Jim Campbell's introductory essay. Campbell summarizes each author's model, prediction, and level of certainty.

All of the forecasts are based on the percent of the two-party vote predicted for the incumbent presidential party, so the predictions are for McCain's share of the two-party vote.

A couple of things are worth noting. First, there is quite a range in predicted outcomes, from a low of 41.8% (Brad Lockerbie) to a high of 52.7% (Jim Campbell), with a median forecast of 48.0% for McCain. My own national forecast (my state-level model depends upon September polls and is not yet ready) is for McCain to get 44.3% of the two-party vote, so I'm a bit on the low end. Second, while there is a lot of diversity among the models every one except Norpoth's includes at least one retrospective variable, such as presidential approval or some measure of economic performance.

So, what should we make of these predictions? One interesting finding from the last several years of election forecasts is that while there is a lot of variation in error across models in any given year, almost every forecasting model has predicted correctly the eventual winner of the popular vote. In fact, going back as far as 1996 I calculate that leading forecasters (mostly published in PS, as well as a couple of other prominent models) called the correct popular vote winner in 27 out of 28 cases . In addition--and I'm not sure how to explain this, or what type of bias it might imply--in 25 out of 28 cases, the models overestimated (sometimes slightly, other times by a wide margin) the share of the vote going to the incumbent party. These two findings, combined with the direction of the forecasts posted above, would seem to suggest dim prospects for the McCain campaign.

For what it's worth McCain's share of the two-party vote in current polls is 46.4%.

1 comment:

Melisa Marzett said...

Your forecast is trustful:) These two findings would seem to suggest dim prospects for the McCain campaign! Use when you need to improve your articles!