Cell Phones vs. Landlines

I've heard, and had, numerous discussions about polls and whether the growing number of people who use only cell phones has caused a problem with accuracy. I use only a cell phone, as do all my children and many of my friends. The concern is that pollsters use landlines in order to gather information. How does this impact the polls we see? Current polling in the 2008 presidential election shows a very tight race between Democrat Barack Obama and Republican John McCain. In part because of the strong support Obama is attracting among younger voters, and as the number of Americans who are reachable only by cell phones rises, interest continues to grow in the question of whether public opinion polls that do not include cell phones are accurately measuring the relative levels of support for the two candidates. The Pew Research Center for the People & the Press has conducted three major election surveys with both cell phone and land line samples since the conclusion of the primaries. In each of the surveys, there were only small, and not statistically significant, differences between presidential horserace estimates based on the combined interviews and estimates based on the land line surveys only. Yet a virtually identical pattern is seen across all three surveys: In each case, including cell phone interviews resulted in slightly more support for Obama and slightly less for McCain, a consistent difference of two-to-three points in the margin. What you see may not be what you get in this case. Take your favorite poll and add 2 to 3 percentage point for Obama. Could we be looking at a 10% win for Obama?