Friday, September 5, 2008

Convention Bump Recap

Now that we have a little distance from the end of the Republican convention it's possible to compare the actual convention bumps to those I had anticipated. As described in a previous post, the bump is calculated using Campbell, Cherry, and Wink's (1992) method: the candidate's average share of the two-party vote in trial-heat polls conducted six days to two weeks prior to the start of the convention is subtracted from his share of the two-party vote in polls conducted during the seven days following the close of the convention. It is important to bear in mind that this is only one way to gauge convention bumps and it does not say anything about the duration of the bumps.

Based data from polls* collected from August 29 to Septemeber 3, I calculate that Obama received a 1.8 percentage point convention bump. This is a trivial increase, running below both the average historical (1964-2004) bump of 5.9 points, as well as my prediction of a 5.5 point bump. Using polls collected from September 5 to September 11, I put the McCain bump at a slightly higher 2.1 points, still relatively modest by historical standards but slightly higher than my prediction of 1.4 points. Both candidates enjoyed larger bumps in the first few days after the conventions but support averaged out to much smaller bumps by the end of the first week after the conventions.

While the error in predicting the McCain bump is relatively small, the Obama prediction error is appreciably larger. The most likely explanation lies in the fact is that the Republican VP announcement took place the day after the Democratic convention and the Republican convention started (sort of--Gustav) the following Monday. In effect, Obama/Biden disappeared from media coverage the day after the convention, in contrast to a period of post-convention media coverage that typically (I think) tilts toward the convening party. It was also probably a mistake to wait until the Saturday prior to the convention to announce the Biden selection. Had this been announced on Wednesday or Thursday of the week before the convention, the Democrats would have gotten a couple of extra days of positive and abundant coverage heading into the convention.

One final explanation that has to be considered is that the upper limit of Obama's potential support could be lower than anticipated. Recall that my "bump" model controls for how far ahead or behind the predicted outcome the candidate is running prior to the convention. Based on presidential approval, aggregated satisfaction with person finances, and an open seat contest, my model predicts Obama winning with 55.7% of the two-party vote (seems a bit of a stretch at this point, but lets wait until election day to see how close it is). To the extent that this prediction is higher than his real potential vote, the bump model will have over estimated his bump. At the same time, though, the bump model should have underestimated McCain's bump by a similar margin, which it didn't. In the end, I suspect that all three factors may explain Obama's under-performance.

*Polling data are taken from ,
, and

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