Brewers!

C.C. gets the last out: Brewers 3, Cubs 1



The Brewers watch the end of the Mets/Marlins game on the Jumbotron. Go Fish!


Marlins win, Brewers celebrate!


It's nice to take a break from the election.

Debate Effects

The first of three presidential debates is scheduled for this Friday, the 26th of September. We can expect to see the two campaigns positioning themselves and trying to manipulate (lower) expectations in the coming days, and we can also expect the media to focus on how pivotal the debates might be to determining the contours of the remaining weeks of the campaign. These things will happen because of the high visibility nature of these one-night events and the relatively high stakes if one side or another does a lousy job.

But do debates really have much of an impact on the campaign? Are elections won or lost because of debate performances? Is there much of a debate bump? In Do Campaigns Matter? I argued that debates have relatively limited effects because they occur relatively late in the campaign, when opinions are less pliable, and because--unlike conventions--the information flow is relatively balanced, with both sides making their case.

As the debate period draws near, it is useful to look at data from past debates to get a sense of what to expect this year. In the figures and table below I use Wlezien and Erickson's "poll of polls" to gauge relative support for the two major party candidates during the last sixty days of the campaign. The poll of polls measures the percent of the two-party vote for the incumbent party candidate in the latest available public opinion polls for any given date. These data allow me to examine the flow of public opinion during the fall campaign (figures) and also to measure candidate support in discrete periods of time before and after debates (table).

In the figure below every diamond-shaped dot represents a daily polling observation from the poll of polls; the smoothed line is a lowess estimate intended to give a better representation of the "bump and wiggle" of candidate support; and the vertical lines represent the dates on which debates occurred.

Theses figures provide a visualization of the debate effects and there are a few debates the "look" like they mattered. However, some of these visuals could be a bit deceptive because most of the x-axes only span about ten percentage points and also because some of the debates are closer together than others. The table below summarizes the information from the graphs in a way that gives us a firmer grip on the magnitude of the debate bumps. Here, I compare the polling average from six days prior to the debate to the day of the debate with the average from the seven days following the debate.


These data suggest that the norm is for very little swing in candidate support following debates. Across all thirteen presidential debates the average absolute change in candidate support was 1 percentage point. There are a few notable exceptions, of course. Two that stand out are the second debate in 1992, following which George H.W. Bush lost 2 points, and first debate of 2004, after which George W. bush lost 2.26 points. Other debates with above average ( but still small) vote shifts are the first debate in 1996 and the second debates in 1988 and 2000. Each of these debates has its own story, and I'm sure we can all think of anecdotes to explain the bumps and wiggles. Although the analysis is terribly outdated by now, the debate model from Do Campaigns Matter? came to the profound conclusion that the candidate viewed as having won the debate generally gets a small bump (I told you it was profound).

Focusing on single debate bumps may be obscuring a more general, cumulative effect of debates. The last column in the table shows the change in candidate support from one week prior to the first debate to one week after the final debate. Here we see that the debate period generated a 2.42 point bump for George H. W. Bush in 1988, cost Al Gore 3.52 points in 2000, and cost George W. Bush almost 2 points in 2000. Of these, the 2000 debates stand out as the most important, especially in the context of the closeness of the election. Part of the explanation for Gore's swoon during the debate period is perhaps related to his performance but another important factor was the media meme that emerged as a result of the debates, including open discussions of whether or not Al Gore was a "serial exaggerator" (see Jamieson and Waldman).

So what does this mean for the three presidential debates this year? I don't expect to see large swings in candidate support following the individual debates, barring something really spectacular happening. However, even relatively small shifts in the same direction over the three debates could make this relatively tight race even tighter (if the shifts favor McCain), or could blow it open (if the shifts favor Obama).

Note: I've made a minor change to the title of the table since the original post.

Media and Momentum

In my last post I mentioned that I thought part of the explanation for Obama's meager convention bump was that the Palin announcement on the day after the convention robbed Obama/Biden of much of the post-convention media coverage that might otherwise have benefited them. The more general thought along these lines is that convention bumps are the product of candidates dominating news cycles for a period of several days. This is what I found when I wrote Do Campaigns Matter? and I suspect the same holds true today.

The data below illustrate the ebb and flow of media coverage from early August through mid-September (the dots represent daily observations and the line is the smoothed--lowess--trend estimate). These data do not capture the tone of coverage but rather the extent to which one candidate or the other dominates news coverage. It's possible, of course, that more coverage could be due to negative stories, but it strikes me that the coverage around the conventions is mostly beneficial to the convening party.


There are a couple of important observations to draw from this figure. First, Obama's media presence peaked during the Democratic convention and then bottomed out immediately after the convention's end. It certainly seemed at the time that Obama and Biden disappeared following the the convention, and these data show that that was the case. On the Republican side McCain enjoyed a media advantage during the Republican convention and generally has managed to maintain a slight edge in coverage in the weeks since the convention. Although McCain's advantage is slight, he was able to extend the level of coverage from the convention for a longer period than was Obama.

The other interesting finding here is the degree to which Obama dominated media coverage in the pre-convention period. During the period from August 1 to August 24 Obama averaged 57.4% of all candidate news mentions; and with the exception of a single day (August 15) Obama led in news mentions during the entire period. This lends some credence to the McCain campaign's refrain from earlier this summer that the media were providing much more coverage of Obama than McCain. However, whereas Obama clearly drew more coverage in the pre-convention period, the post-convention period is more balanced.

Note: the data for media coverage come from perspctv.com, a web-based news and information aggregater.

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 Pollster.com , realclearpolitics.com
, and pollingreport.com