The graph above plots the standard deviation of each week’s national likely-voter polls using the midpoint of the fieldwork period as the basis for classification. Values before about mid-August are based on small numbers of polls and are thus more noisy. Starting with the week of September 9th, there are at least 10 polls included in each week’s estimate.
As expected the variation across pollsters has diminished as we get close to Election Day. Over the past week the standard deviation of the estimates for Obama and Romney support, and for the margin between them, has converged to about 1.1-1.2, about a third the value seen during the month of September.
Estimates of support for President Obama have shown more variability than those for his opponent. The average standard deviation over this period was 1.95 for the President compared to 1.56 for Governor Romney. Naturally the lead shows much more variability than the estimates for either candidate with an average value of 2.73.
Critics of Rasmussen’s polling suggest his results converge with the results of the other pollsters as Election Day nears. If we plot his polls against the average for all other pollsters, we find less evidence for convergence than the critics allege.
While it is obvious that Rasmussen’s polls ran more Republican than the consensus, as I have shown before, there is still a substantial gap of about two points between his results and the other pollsters. His most recent figures put the race as tied nationally while the consensus has President Obama ahead by somewhat over two percent.