Beware of Poll Averaging

The Wall Street Journal runs an interesting piece warning about web sites such as Real Clear Politics and Pollster.com that publish polling averages. The technique is used to "smooth out aberrant results" but their methods "are criticized by statisticians."

"Among the pitfalls: Polls have different sample sizes, yet in the composite, those with more respondents are weighted the same. They are fielded at different times, some before respondents have absorbed the results from other states' primaries. They cover different populations, especially during primaries when turnout is traditionally lower. It's expensive to reach the target number of likely voters, so some pollsters apply looser screens. Also, pollsters apply different weights to adjust for voters they've missed. And wording of questions can differ, which makes it especially tricky to count undecided voters. Even identifying these differences isn't easy, as some of the included polls aren't adequately footnoted."

While averaging polls may help people make sense of conflicting poll results, the technique usually introduces other biases that make the average just as unreliable.


February 24, 2008


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