  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.
