A Democratic reader writes:
Fox News changed its sampling. Last week - D43, R 37 This week - D41, R39 http://courses.ttu.edu/hdfs3390-reifman/weighting.htm There is no good reason to change the weighting, they just did it and not surprisingly came out with a more favorable result for McCain.Small variations in weighting can have proportionately large effects on the perception of a poll, t'is true.
But I'm no expert on this question, so I asked Mark Blumenthal of Pollster.com what he thought.
He tells me: "If they have been weighting by party, and they suddenly changed their weighting, I'd say it's a pretty questionable practice. The last four Fox surveys all varied slightly in terms of party ID, so I'm not sure that's what they've done. That said, if the other surveys this week show no similar "shift" in party, then it's probably reasonable to discount this result."
"Every day, we have 8-12 new national polls. Some go up, some go down, some by a little, some by a lot. Remember, by chance alone, we should expect at least 1 in 20 to produce a result outside the 'margin of error.'"
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