What a Convention "Bounce" Looks Like

very interesting article

The convention bounce is one of the more vexing problems in presidential forecasting. That is particularly so this year. Both conventions are being held late -- the Democratic convention dates are the latest since at least 1952 for the challenging party, and the Republicans' are the latest since at least 1952 for the incumbent party. Moreover, the conventions are being held right on top of one another, with just one week of separation between them. Most years, there is at least a three- or four-week gap between the opening dates of the conventions. That gap was as small as 14 days in 1996 and 15 days in 2000, but it has never been so small as the seven days that we have this year.

But the convention bounce is also not something that can simply be ignored. If we looked at the polls from the weekend immediately following the Democratic convention, for instance, and did not adjust them in any way, that might give us an inflated idea about Barack Obama's chances of occupying the White House.

What I have done, then, is to boot up my database of national polls, which covers all elections since 1952. I looked at the average margin for all polls conducted within 150 days of the election; for instance, the average of all polls taken within 150 days of the election in 2000 had George W. Bush ahead by 2.0 points. Then, I looked at to what (if any) extent the candidate was overperforming that average in the 21 days commencing from the first date of his party's convention. For instance, if GWB polled 9 points ahead of Al Gore in a survey conducted one week from the start of the Republican convention, that would be considered evidence of a 7-point bounce -- his 9-point lead in that poll less his long-term, 2-point advantage.

Plotting all such polls produces a graph like this:

www.fivethirtyeight.com