Our daily Slate Scorecard update posted earlier this evening focuses on the new poll from Mason-Dixon that shows a narrowing race in the Virginia Senate pitting incumbent Republican George Allen against Democratic challenger Jim Webb.We also discuss why the Slate Scorecard does not include the online Zogby Interactive/Wall Street Journal polls and made the following observation:
The latest Zogby results for Virginia-showing Webb ahead 50 percent to 43 percent-help explain our caution. Zogby's Virginia samples have been consistently more favorable to Webb than other pollsters, suggesting a bias in Zogby's online methodology.
With the help of Charles Franklin, here is a chart showing the consistent difference in Virginia. It plots the Allen margin (that is, Allen's percentage of the vote minus Webb's percentage -- click on the graph for a full size image) for each of the four pollsters that have tracked the race. All four show the same sharp drop in Allen's lead since July, but the Zogby result (the green line) has been consistently more favorable to Webb than the three telephone pollsters that use random probability samples of all telephone households rather than samples of Internet volunteers.
The differences are not trivial. The latest polls from Survey-USA, Mason-Dixon and Rasmussen have shown Allen with leads of 3, 4 and 5 percentage points respectively. Zogby's result is very different, showing Webb with a seven percentage point lead.
Incidentally, we do include all of the Zogby Interactive results and other Internet polls in the charts and tables here on Pollster.com. Our aim is to give readers the ability to compare results across pollsters. One minor wrinkle -- at least for now -- is that the 5-poll averages reported here may differ from those on Slate because the averages here include the Zogby numbers while those reported on Slate do not. We are hoping to address that conflict in a future update to our chart pages.
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