David Moore | December 1, 2008
The final presidential contest predictions of the major media polls all came pretty close to the actual results, predicting Obama to win by anywhere from 5 to 11 percentage points (he actually won by 6.7 points).
However, the polls showed a great deal of variability even during the last four weeks of October leading into the election, raising questions about how to measure poll "accuracy" during the election campaign itself.
Shown below is a graph of the results of 10 polls that publicized results for at least the final three weeks of October.
An examination of the daily tracking polls provides no better picture of poll accuracy.
The differences in the overall trends are quite substantial, as are individual points.
On October 12, IBD/TIPP shows an Obama lead of two points, while DailyKos says it's 12. The next day, IBD/TIPP produces a 3-point lead, while GWU has a 13-point lead. On October 25, GWU's lead is just three points, while DailyKos has it at 12 points. Even right before the election, IBD/TIPP shows just a 2-point lead, while ABC/WP says the lead is 11 points.
More important are the many different pictures of the dynamics of the race. If we single out, say, DailyKos from GWU, one would never know they are measuring the same contest - except that they both converge in their final predictions. Another example: Rasmussen shows only a little variability in the race, between an Obama lead of 3 to 8 points, ending at 6, while GWU goes from 13 points down to 1, finally ending at 5. Likewise, Zogby's description of the campaign dynamics shows a relatively stable race for the first half of the month, followed by a major surge, a big decline, and then a last minute surge. DailyKos usually had the most optimistic Obama leads, mostly double digits, except for the middle of the month, and then at the end when the lead declined to just five points.
What can we say about poll performances when there are such different stories about the October dynamics? The notion that the polls were mostly "accurate" must be modified to reflect how divergent they were during the campaign.