The Pew Research Center, which has been at the forefront of efforts to measure the impact of "cell phone only" households on political surveys, has a new report out on subject today. Like Gallup they have found evidence that including interviews of cell-only Americans as a "modest affect" on results in the presidential race:
Pollsters are continuing to monitor changes in telephone use by the U.S. public, since most surveys are still conducted using only landline telephones. Growing numbers of Americans are reachable only by cell phone, and an even larger number who have both a landline and a cell phone may be "functionally cell-only" because of their phone use habits. The latest Pew Research Center national survey, conducted June 18-29 with a sample of 2,004 adults including 503 on a cell phone, finds that the overall estimate of voter presidential preference is modestly affected by whether or not the cell phone respondents are included. Obama holds a 48% to 40% lead in the sample that includes cell phones, and a 46% to 41% advantage in the landline sample. Estimates of congressional vote are the same in the landline and combined samples. [Emphasis added].
The numbers noted above are based on interviews with registered voters. When they narrow the universe to more likely voters, however, the difference [mostly] disappears:
Narrowing the analysis to voters who are certain about their vote choice, there is almost no difference between the landline and combined samples: Obama has a 38%-28% advantage in the combined sample, while the margin is 38%-30% in the landline sample.
[I added the word "mostly" to my second paragraph based on comments below. The report does not speak to the statistical significance of either set of numbers].
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