"Bounce" or "Bump?" The terminology is up to you, but this is certainly the season to consider the short term changes in polling numbers that frequently result in the wake of national political conventions.
A useful first stop would be the 2004 analysis from Gallup's Jeff Jones. It includes the post convention "bounce" numbers from Gallup back to 1964 that are the primary source of the conclusion that average post convention gain for candidates has been six or seven percentage points.
Last week, Tom Holbrook posted a more a thorough review of the past "before and after" data and it's implications. He looks at the gain for each candidate by taking their "average share of the two-party vote in trial-heat polls conducted six days to two weeks prior to the start of the convention" an subtracting that from the candidate's "share of the two-party vote in polls conducted during the seven days following the close of the convention." He does not list the polls used for each year, but presumably his data looks at much more than the Gallup time series for more recent elections.
Holbrook's post is worth reading in full for the lessons he draws from the considerable variation in past convention bumps, although probably the most important is his caution that "the magnitude of the convention bump is not a great predictor of election outcome." Still, he sees a pattern to the past variation that "should be a useful guide to what to expect" from the conventions and promises to update later this week with a prediction for each candidate.
But before reading two much into the twitches in the daily tracking polls over the next three weeks, please read the latest column from CBS News director of surveys Kathy Frankovic. She reminds us that the gap between the Democratic and Republican conventions is just three days -- much shorter than in past elections -- and will coincide with the Labor Day weekend:
Will we even be able to measure whatever impact the Democratic Convention has on Obama before it’s time to measure the GOP convention’s impact on John McCain? And will we be able to sort out what has caused what?
Will we actually discover a “bounce” or a “bump?”
Probably not. Polling over Labor Day Weekend is always a problem. We confront more than the usual number of people who don’t respond or can’t respond. People are away from their homes, heading back from summer vacation, or preparing their children for the start of the school year. In addition, the focus will shift so quickly from the Democrats to the Republicans that whatever opinions might be expressed over Labor Day Weekend might not last too long.
I thought it would be helpful to look at some past elections, not in terms of the immediate "before and after" averages, but rather where the conventions fit into the longer arc of the trend in the vote preference over the course of the election year. I spent some time gathering past polls from a variety of sources and asked Charles Franklin to create some charts matching the format he used to look at the trends of the 2000 and 2004 elections a few weeks back.
I had hoped to use those charts for a series of posts. Unfortunately, I got delayed, so I will post the charts along with a some very compressed discussion after the jump.
The chart and all that follow plot the trend in the margin separating the top two candidates in polls over the course of the year. As with our standard charts, we plot each poll result as a dot and then plot a regression trend line though the dots. In this case, Franklin added "educational confidence intervals" around the trend line.
You will notice a pattern in 1980 that is evident in other years. Six surveys conducted after the Republican conventions showed Reagan with huge leads of 16 or more percentage points, but these quickly faded after the Democratic convention in August. The see-saw between July and late August is so abrupt relative to the number of surveys that the trend line never quite catches up.
One of the striking aspects of this chart is how much the volume of polling has increased over the last 28 years. In searching the Roper Center Archives and also the volume of results published by Gallup, I found only 58 non-overlapping horse race results over the course of the year. It is late August, and we have already logged 239 such surveys so far in 2008.
Note also the significant change in convention coverage and ratings over the last 30 years. In 1980, the three major networks devoted their full prime time schedules to coverage of the conventions. Now, an hour a night is the norm, and the percentage of American households tuned to the major networks is roughly half of what it was in 1980.
The two bumps of 1984 were smaller than in other years in Holbrook's time series and in the trend lines on this chart. Just two two surveys conducted immediately after the Democratic convention showed a much closer race than most of the other surveys conducted just a few weeks later. The 1984 Republican convention helped boost Ronald Reagan's margins to the 15 to 25 point margins he held throughout that reelection campaign.
Incidentally, we obtained the data plotted for 1984 and 1988 from data trove the now-defunct publication Public Opinion.
In the non-incumbent race of 1988, Michael Dukakis received a healthy boost from the Democratic convention, but it hat all but disappeared before the Republican convention in August. In contrast, the boost provided to George H.W. Bush by the Republican convention persisted throughout the fall election.
The 1992 Democratic convention delivered to Bill Clinton what is by far the biggest and most lasting boost to any nominee in recent history. Ross Perot's temporary departure from the race at the end of the Democratic convention (he returned by October) helps explain part of the dramatic shift (as well as the reason for four polls showing a closer race in the midst of the Democratic convention.
However, it is a mistake to see this shift only as an artifact of Perot's exit. Here is a bonus look at the change in Bill Clinton's favorable rating as measured by Gallup during 1992. Perceptions of Clinton shifted from net negative (41% favorable, 49% unfavorable) just before the Democratic convention to net positive (averaging 55% favorable, 37% unfavorable) throughout the rest of the fall campaign.
Thanks to Andrew Gelman for sharing the data plotted in the 1992 chart.
Update: An earlier version of this post omitted the chart for 1996 and included charts for 2000 and 2004 in a slightly different format. The charts below match those from 1980 through 1992.