Why big difference in gubernatorial polls?

thumbsupdownA new poll from the research firm YouGov, in partnership with the New York Times and CBS News, has Gov. Sam Brownback ahead of his Democratic challenger, House Minority Leader Paul Davis, by 10 points, 47 to 37 percent. Yet SurveyUSA polls, sponsored by KSN, Channel 3, have consistently shown Davis ahead, including one released last week that had Davis up 8 points, 48 to 40 percent. How could there be such a big difference? The YouGov poll is using a new sampling method based on an online panel, while SurveyUSA uses the traditional method of random telephone surveys. The non-probability online panel is controversial, and it tends to underrepresent minorities (only 4.4 percent in the Kansas poll). Ultimately, the only polling that matters is what happens in the voting booth.