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Update: Cell Phones & Coverage Bias

Topics: Cell Phones , Coverage error , Nate Silver , Pew Research Center , Scott Keeter , Weighting

Following-up on my post on Monday, which discussion of Nate Silver's suggestion that pollsters consider weighting on non-traditional variables, the Pew Research Center's Scott Keeter emails to point out they did a post election analysis that suggests strong potential for adding variables like income, marital status and home ownership to reduce the bias assess the effect of adding additional variables to their weighting scheme.

They found that the additional variables reduced the cell-phone-only bias substantially, though as Keeter points out, "this was not a comparison of different weighting schemes, but was instructive of what additional weighting factors might accomplish."

Here is the money graph (and table) from Pew's "Perils of Polling in Election 2008" report:

Logistic regression was used to estimate the probability of voting for Obama among landline voters and cell-only voters. As would be expected, the difference is sizeable; the predicted probability of voting for Obama is 16 points higher for cell-only voters than for landline voters. Adding most of the standard demographic variables used in weighting (e.g., age, sex, race, Hispanic [ancestry], education, and region) to the model (labeled the "standard model" in Table 3) reduces this difference to 11 points, a result consistent with the notion that weighting helps reduce but not eliminate the potential for non-coverage bias. Including income, marital status and home ownership in the model reduces the difference even further to 5 points. When these additional demographics are included in the model, being cell phone only is no longer a significant predictor of candidate support, as it was in the first two models.

2010-05-28-Pew-Analysis.png

 

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