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Thoughts on the Gallup Controversy

September 29th, 2004, 12:00pm by Sam Wang

Relevant to the current Gallup controversy: Here is a table of Gallup national presidential polls, along with Party ID statistics for each poll. The GOP-Democratic margin in the poll correlates quite closely with the Bush-Kerry margin. In fact, the correlation coefficient is 0.73 (r2=0.53, P<0.001). Put into lay terms, this means that Party ID gap and Bush-Kerry margin vary together, and variation in one can account for over half the variation in the other. A linear fit between the two is near 1: on average, every extra Republican in the sample added one to Bush’s margin.

This seems consistent with the idea that how much Gallup samples from each group (Republicans, Indepdendents, Democrats) affects poll outcome. But could it be the other way around: could voter sentiment affect self-reported party ID? One test of this is to see if the group sizes fluctuate as much as would be expected by chance. For a sample of 1000 voters composed of 39% R, 34% D, and 25% I (the average of all those polls), the percentage of R’s would be expected to have a standard deviation of 1.5%. The actual SD is almost twice as large, 2.9%. Therefore party ID does vary more than sampling error would suggest. Maybe it varies with sentiment. Or maybe conditions at Gallup change (for instance, the time of day and week that calls were made). Hard to know. One thing is clear: their average party ID breakdown does not match known values (35% R, 38-39% D, 26% I).

Gallup Party ID bias

Tags: 2004 Election

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