Evidence of Systematic Bias in 2008 Presidential Polling (preliminary report)

Abstract

Political polls achieve their results by sampling a small number of potential voters rather than the population as a whole. This leads to sampling error which most polling agencies dutifully report. But factors such as nonrepresentative samples, question wording and nonresponse can produce non-sampling errors. While pollsters are aware of such errors, they are difficult to quantify and seldom reported. When a polling agency, whether by intention or not, produces results with non-sampling errors that systematically favor one candidate over another, then that agency's poll is biased. We analyzed polling data for the (on-going) 2008 Presidential race, and though our methods do not allow us to identify which agencies' polls are biased, they do provide significant evidence that some agencies' polls are. We compared polls produced by major television networks with those produced by Gallup and Rasmussen. We found that, taken as a whole, polls produced by the networks were significantly to the left of those produced by Gallup and Rasmussen. We used the available data to provide a tentative ordering of the major television networks' polls from right to left. Our order was: FOX, CNN, NBC (which partners with the Wall Street Journal), ABC (which partners with the Washington Post), CBS (which partners with the New York Times). These results appear to comport well with the informal perceptions of the political leanings of these agencies. Our findings are preliminary, but they make a case for further research into the causes of and remedies for polling bias.