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Did Including Cell Phone Only Respondents Increase Polling Accuracy?

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Now that most of the national vote has been tabulated, we can get a pretty good sense of which pollsters came closest to pegging the final popular vote. As Mark mentioned in an earlier post, several others have done this already, but I thought I'd create these plots for Pollster.com readers.

The final national poll results from individual pollsters are plotted below (these are the last 19 national polls listed on Pollster.com's national trend page). The pollsters represented with red dots are those that included cell phone only (CPO) respondents in their sample. The Obama vote is represented by the y-axis and the McCain vote is the x-axis (UPDATE: I've updated the plots to reflect the updated vote share of 52.5% for Obama and 46.2% for McCain). The horizontal red line is the actual vote that Obama garnered while the vertical line indicates McCain's share of the vote. The closer a poll is located to where the two red lines meet, the more accurate that poll was in predicting the final outcome.

accuracy1b.PNG

Note that every pre-election poll plotted here underestimated McCain's support. However, the big winners were Rasmussen and Pew, both of whom estimated a 52-46% advantage for Obama. The Pew poll included CPO respondents while the Rasmussen survey did not. CNN and Ipsos/McClatchy also came quite close by estimating a 53-46% advantage for Obama (UPDATE: These polls now come just as close as the Pew/Rasmussen polls). Neither survey reached the CPO population. Indeed, the plot reveals no clear pattern with regard to the CPO issue. Polls including CPO respondents did not appear to be any more accurate than those only reaching landlines.

Before the election, I separated out the Pollster.com national trend into surveys including the CPO population and those who were only calling landlines. The plot below looks at how each of these trends performed.

accuracy3b.PNG

The trend based on surveys including the CPO population did slightly better at estimating Obama's vote but worse at gauging McCain's support. Overall, the CPO trend was slightly further off the mark than the landline trend.

Of course, there are any number of other factors at play with these different surveys (such as different likely voter screens, weighting, etc), so we can't draw any definitive conclusions from this analysis. But there is no obvious pattern from these initial results that indicate that including CPO respondents helped improve polling accuracy.

Update: Updated to reflect changes in popular vote.