Analysis of Zogby State Tracking Polls
In a previous posting, I pointed out that there was no consistency in the day-to-day movements of the (then) four national tracking polls, suggesting that these shifts were basically random and that the underlying division of the electorate between Bush and Kerry was not changing. Since then, political junkies like me have discovered yet another set of tracking polls to obsess about--the Zogby 10 state tracking poll.
Now I respect John Zogby, even though he is considered something of a pariah in the polling community for, among other things, his use of a rigid party identification weighting scheme in his national polls. But four years ago, Zogby also undertook a series of state tracking polls in the final days of the campaign and the results were decidedly mixed. Some of Zogby's final state polls were very accurate but some were wildly inaccurate. The most notable example of the latter was Zogby's final poll in California which showed Al Gore leading George Bush by a meager 1 percentage point. On Election Day, Gore swamped Bush in the Golden State by 12 percentage points.
So based on his track record, there are reasons to be skeptical about the results of Zogby's 2004 state tracking polls. For example, Zogby's current tracking poll shows George Bush leading John Kerry by 9 points in New Mexico. This is simply not plausible. Al Gore narrowly carried New Mexico in 2000 and since then the Hispanic proportion of the New Mexico electorate has increased. For some reason, perhaps a lack of Spanish-speaking interviewers, the Zogby tracking poll is clearly off the mark in New Mexico.
But the main point that I want to make here is not that some of Zogby's state polls are probably inaccurate, but that the day-to-day movements of these polls across the ten states are totally inconsistent. There have been rather dramatic shifts in the relative standing of the candidates in some of these states, but these shifts show no consistent pattern. For example, since Zogby released his first tracking poll results on October 24, George Bush has gained 9 points of margin in Colorado and Michigan, but John Kerry has gained 11 points of margin in Wisconsin and 5 points in Florida. In just the past two nights, Bush has gained 8 points in Ohio and 6 points in Colorado. Pretty scary, right? But in the same two nights, John Kerry has gained 5 points in Wisconsin, 4 points in Minnesota, and 3 points in Florida and Michigan.
The average correlation in the day-to-day movements between each state and the other nine states ranges from -.16 for Michigan to .02 for Pennsylvania. The average correlation across all 10 states is -.08.
Since October 24 the mean shift in the margin between Kerry and Bush across the ten states has been exactly zero. In the past 2 days, the mean shift has been 0.2 points toward Kerry.
Conclusion--there is nothing going on here. The day-to-day shifts in the Zogby state tracking polls, like the day-to-day shifts in the national tracking polls are basically random. So stop obsessing. Do something useful. Go carve a pumpkin and make sure that everyone you know who hasn't already voted gets to the polls on Tuesday.