Pew Research Center for the People & the Press.
Available online.
"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... finds that the overall estimate of voter presidential preference is modestly affected by whether or not the cell phone respondents are included."
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