THE BLOG

Survey Practice on the "Convergence Mystery"

12/22/2008 11:14 am ET | Updated May 25, 2011

Survey Practice, AAPOR's online publication, has a new issue out that includes a follow-up to David Moore's provocative piece on what he dubbed the "convergence mystery." Moore observed that the national polls often produced "contradictory estimates and trends" -- and more specifically, a greater variance of results -- that significantly converged over the last few days of the campaign. I posted a follow-up here that showed a similar pattern in state level polling and posed a theory that might explain some of the convergence, and Moore responded with more data and a different theory.   

In their new issue, Survey Practice publishes responses from five survey researchers who deserve the title "expert:" Paul J. Lavrakas, Michael Traugott, Micheline Blum, Cliff Zukin and Don Dresser. David Moore follows up with a new article of his own that begins with this helpful summary:

The two major types of explanations offered by our experts for the convergence of polls right before the election are based on 1) changes in the pollsters’ methodology and 2) changes in the certainty of the vote choice.

The first explanation suggests that in the final weeks of the campaign, many pollsters adjust their likely voter models (mentioned by Lavrakas and Blum) or they increase their sample sizes (Dresser).

Lavrakas argues that the adjustment of the voter models, even when done explicitly to make their outcomes more consistent with other polls, should be seen as a positive action rather than as a “suspicious” activity. However viewed, such last minute changes could account for some of the convergence.

Dresser mentions the tendency of many pollsters to substantially increase their sample sizes for their final pre-election polls, to insure as small a margin of sampling error as they can reasonably afford. He specifically mentions Pew, Harris and ABC/Washington Post.

Blum suggests that the outlier polls during the campaign were either a) less likely to poll in the final three days, or b) more likely to allocate their undecided voters, thus bringing them closer to the mean.

Zukin speculates that the convergence has less to do with pollsters’ methods and more to do with the “phenomenon we are measuring.” Specifically, as voters become more certain about their choices, polls will tend to converge toward each other. That sentiment is also found in the explanations by Lavrakas and Blum.

Moore goes on to offer his own thoughts and asks why polls, "conducted at the same time, using virtually the same wording - [are] supposedly more accurate (reliable) when opinion is more crystallized?" For those intrigued by the "mystery," It is all worth reading in full.

Among all of these efforts to explain, I was most intrigued by Blum's analysis, which was the only one of the five to scrutinize the poll data directly. Among the national polls, at least, much of the variance in mid-October can be explained by a small handful of "outlier" results two-weeks out from the election:

Of the six organizations with outlier polls, three reported margins larger than 11 points, and all three consistently showed larger margins in their polls. In the final three days, however, one of these organizations allocated the undecided, giving more to McCain, one did not release a poll, and one had a margin of exactly11 points on its final poll. Of the three organizations releasing four polls with margins smaller than 5 points, one organization (with 2 “outlier” polls) allocated the undecided in the final three days, one did not release a poll in the final three days, and one “converged.” So, of the 6 organizations, 2 organizations (accounting for 3 “outlier” polls) allocated undecided in the final three days in the direction of the previously underestimated candidate, 2 did not release polls in last 3 days, and only 2 “converged.” If we remove the 7 “outlier” polls from the 29 released in the week of 10/21-27, the variance is reduced to only 2.9 points.

Her conclusion:

Basically then, both of the explanations examined, the allocation of the undecided by seven organizations in the final three days and the absence or favorable allocation of a few “outlier” organizations, appear to be major contributors to the “convergence” seen. Apportioning the undecided in the favorable direction and the absence of previous outliers virtually guarantees less variance and the appearance of “convergence. So, perhaps, rather than convergence, what we saw was that much of the earlier variance was due to a few outliers and that the final three days benefited from their absence or favorable apportionment of their undecided vote.

Blum's observation prompted me to take a closer look at the national polls that allocated the undecided. I find seven projections by six organizations (the two pollsters for the GWU/Battleground poll, Republican Ed Goeas and Democrat Celinda Lake, once again produced competing projections with different allocations). The process of allocating undecided should not, by itself, raise any suspicion. Of the seven allocations, three changed the initial margin not at all or within rounding difference of the averages, two moved the margin farther away from the poll averages as of Monday 11/4, and one moved it closer. However, that one move -- by the IBD/TIPP poll -- was important in helping produce the reduction in variance that Blum describes.

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