Articles and Analysis


Polling Trends in 2008 vs '04 and '00


The most common description of polls is that they are snapshots, not predictions. A good way to look at that in the 2008 election is to compare the '08 campaign with the two that came before.

The chart above shows the trend estimates for each of the last three presidential campaigns. I'm plotting the estimated margin between the two candidates, Dem minus Rep, for each year.

With 93 days to go until the 2008 election, Obama holds a 3.3 point advantage over McCain, though that has been eroding over the past six weeks. If we put a confidence interval around today's estimate, we get a race that is just barely leaning Democratic.

But what about the future? The dynamics of the next 92 days are all important for where we stand on November 4. Since we can't foresee those 92 days yet, let's see what happened during the same time in 2000 and 2004. That gives us a better idea how much change we might anticipate in the next three months.

In 2004, Kerry slowly built a 2 point lead by this time, and held a small lead through much of the summer. But then the race took a sharp turn, with Bush making a 6 point run, taking a four point lead with 50 days to go. Kerry gained back 3 points of that in the polling, but less than 2 points of it in the actual vote, losing by a 2.4 point margin.

In 2000, Bush led in most of the early polls, holding a 6 point lead with 107 days to go. Then Gore moved sharply up, erasing Bush's lead and then adding a 3 point lead for Gore with about 56 days left. Bush promptly reversed Gore's gains with a six point move in the GOP's direction, and led by about 3 points over the last three weeks of the campaign. Of course, the 2000 polls were misleading in predicting a Bush win. Gore won the popular vote by 0.6 points.

So far in 2008, Obama has enjoyed a run up of 5.5 points since his low point in late March. That run is on a par with Bush's in 2004 but still a bit less than Gore's 9 point run in 2000, and on par the Bush's 6 point rebound that year.

Judging from the dynamics we've seen in the past it is quite reasonable to expect the current trend to shift by half-a-dozen points. August and the conventions have been periods of substantial change in both previous elections, so if history repeats itself the next 4 or 5 weeks should be pretty interesting.

The bottom line is neither campaign should be complacent or despondent. There is a lot of time left and recent history shows that both up and down swings of 6-9 points are entirely plausible.

As a P.S. here are the three campaigns with educational confidence intervals around them.


The current 2008 estimate is just barely inside the "lean Dem" range, and will move to toss up if the current trend continues for another couple or three polls.

The 2004 estimate was pretty close to the outcome which was well within the 68% confidence interval around the trend.

The polls in 2000 were troubling for having the wrong popular vote winner, but even there the outcome was inside the 95% confidence interval. With races as close as the last two, it is worth appreciating just how wide those confidence intervals are.

Our efforts to characterize races rely on the best estimates of those confidence intervals, but it is all too easy to focus on who's ahead and not remember how much uncertainty there is. That uncertainty is both about where the current estimate says the race stands today and about how the race may change in coming weeks. The data here show that unless one candidate builds a bigger lead than either has held so far, the uncertainty remains pretty big.

Note: My trend here is slightly different from the Pollster National trend because I'm working off the difference between candidates, not each trend separately, and because I've made 2008 comparable to 2000 and 2004, just a slightly different amount of smoothing compared to Pollster's standard estimator this year. None of those differences change the qualitative picture or shift the magnitude of changes I cite above.

Cross posted at Political Arithemik.



*That's* the chart I've been waiting for! Adds some needed context for the daily numbers. I hope it can be updated on a regular basis.



What Ciccina said. Thanks Charles.


Gary Kilbride:

The GOP was a 50/50 brand in 2000 and 2004. You're not taking that into account here. McCain's surge potential is severely regulated by 3+ years of Bush stuck at sub-40% approval rating, and how that bleeds to the Republican brand as a whole.

If McCain moves 6-9 points, or anything close to that, it will be relentless implosion by Obama.

I guarantee one thing: The market sites won't move to toss up range, with 2 or 3 more polls, or 10 or 12. That's similar to 2004, when Bush was always the betting favorite despite Kerry maintaining a poll lead for a while. Unfortunately, investors are aware of situational influence, trumping the day to day numerical noise.


Eric Rosenfeld:

There is something that does not look right about the confidence intervals displayed in these plots. The 95% confidence Margin of Error for a poll can be approximated by 100/square root(N) where N is the number of participants in the poll. In the plots displayed in this article there is a much higher density of polls in the final months of the campaigns. Consequently, the total N contributing to the average of the polls should be higher during the final months. Thus the plots should show a narrower margin of error close to the election. However, the plots are drawn with a constant width band of uncertainty. Similarly in the earliest months of the campaign when polls are few and far between, the bands of uncertainty should be wider.

I have been following the group of polls used to compute the averages on RealClearPolitics.com. Recently their list contains polls with an aggregate number of participants around 13,000. When these polls are combined by weighting the polls according to the number of participants in each poll the Margin of Error of the weighted average is based on the combined total number of participants in all the polls. With 13,000 participants this gives a 95% confidence Margin of Error of about +/-.9% for the average support for each candidate. This would correspond to a Margin of Error of less than +/-1.3% on the difference between the candidates' levels of support. This is substantially lower than the margin of error shown in the figures of this article.

A simple equally weighted average of the polls would have a slightly higher Margin of Error.For example, a simple average of 10 polls where each poll has an N of at least 700 would have a Margin of Error less than +/-1.2%. This is no where near the +/-4% margin of error shown in these plots.

My guess is that the 2000 election result would fall outside the revised 95% confidence interval while the 2004 result would fall within the revised confidence interval.


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