>That's true, but you would expect the same sample bias in each tracking
>poll, unless they modified the criteria for likely voters over the course
>of the tracking poll. I suspect that it's merely sample error, accounted
>for in the confidence interval. One should expect the poll numbers to rise
>and fall even if support within the sampled population is stable.
Each pollster uses different techniques to project likely voters, which may account for the diff between Zogby & Gallup. The time trends within each poll are probably the joint product of noise and real volatility.
Doug