If you've been following the polls, after the Republican Convention, McCain seems to have caught up with or overtaken Obama. Apparently this post-convention bounce is not real. How do you create it?
How you do it is to change the composition of your sample. The sample is supposed to represent how the population itself splits (the Democrats have a national 11 million advantage). We are told changes in party affiliation happen very slowly. But the pollsters are moving numbers around.
Emory University political scientist Alan Abramowitz is highly skeptical of the new Gallup, USA Today and CBS polls. About the latter, which showed a statistically insignificant two point lead for McCain, Abramowitz said: "One reason for the dramatic difference between the two recent CBS polls is that the two samples differed fairly dramatically in terms of partisan composition. The first sample was 35.2% Democratic, 26.2 percent Republicans, and 38.6 percent independent. The second sample was 34.9% Democratic, 31.1% Republican, and 34.0% independent. That's a change from a 9 point Democratic advantage to a 3.8 point Democratic advantage. That alone would probably explain about half of the difference in candidate preferences between the two [CBS] polls."
http://www.dailykos.com/story/2008/9/9/18192/04144/290/592615
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