As promised last week, I have a prediction for Obama's bump in the polls from the upcoming Democratic convention.

Based on his current standing in the polls (4.75 points behind his expected--based on my super-duper forecasting model--vote share), the fact that the convention starts seven days before the Republican convention, and accounting for slight decline in convention bumps over time, the convention bump model predicts a 5.5 percentage point bump in Obama's share of the two-party vote in trial-heat polls. This is slightly smaller than the historical average (5.9 percentage points), in part because the two conventions are closer together than any others during the period analyzed (1964-2004).

The prediction is based on the results of a regression analysis (below) of convention bumps, utilizing the variables described above, as well as a dummy variable for the 1968 and 1972 Democratic conventions (I toyed with labeling this the "lousy convention" control variable).

The model makes substantive sense and fits the data fairly well. Of course, there is still a fair degree of unexplained variation in bumps, as illustrated in the graph below. The smallest error was .45 points (Democrats, 1996), the largest was 5.49 points (Democrats, 1984), and the average absolute error was 2.45 points.

What all of this says is that there is a regular pattern to convention bumps and based on that pattern Obama should get a bump of around 5.5 percentage points. However, it is also clear that it is not at all unusual for candidate to over- or under-perform by a few percentage points.

Of course, the fact that McCain's convention begins just a few days after Obama's ends could have an effect on the magnitude of the post-convention bump. While the model controls for the number of days between the beginning of the conventions, there are no cases in the data set where the conventions are this close together. In other words I am predicting outside the range of the data, always a dicey enterprise.

I should have a McCain Bump prediction about this time next week.

## Thursday, August 21, 2008

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