by Scott Keeter, Director of Survey Research, Pew Research Center
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. Barack 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.
Other recent comparisons between landline samples and combined landline and cell samples have found little or no difference in overall results. In a Pew primary election poll in December 2007, Hillary Clinton held a 20-point lead over Obama in the combined sample and a 17-point lead in the landline sample. In a congressional election poll in October 2006 there was no difference in voter preferences between the two samples. Despite the fact that cell-only respondents are often very different from those reached by landline, the relatively small impact from including cell phone samples is a consequence of the statistical weighting applied to surveys as a standard practice among professional pollsters.
The Cell-Only and the “Cell-Mostly”
The number of Americans who have a cell phone but no landline phone has continued to grow, reaching a total of 14.5% of all adults during the last six months of 2007, according to U.S. government estimates. In addition, 22.3% of all adults live in households with both landline and cell phones but say that they receive all or almost all calls on their cell phones.
The cell-only and cell-mostly respondents in the Pew poll are different demographically from others. Compared with all respondents reached on a landline, both groups are significantly younger, more likely to be male, and less likely to be white. But the cell-only and cell-mostly also are different from one another on many characteristics. Compared with the cell-only, the cell-mostly group is more affluent, better educated, and more likely to be married, to have children, and to own a home.
Cell Phones and Voter Preferences
In the current poll, cell-only respondents are significantly more likely than either the landline respondents or the cell-mostly respondents to support Barack Obama and Democratic candidates for Congress this fall. They also are substantially less likely to be registered to vote and – among registered voters – somewhat less likely to say they are absolutely certain they will vote. Despite their demographic differences with the landline respondents, the cell-mostly group is not significantly different from the landline respondents politically.
Yet as Pew has found in the past, when data from landline and cell phone samples are combined and weighted to match the U.S. population on key demographic measures, the results are similar to those from the landline survey alone. Among registered voters in the combined cell and landline sample, support for Barack Obama is two percentage points higher than in the landline sample alone (48% vs. 46%); support for McCain is one point lower (40% vs. 41%). 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.
On two other key political measures, the pattern is similar. Voter intentions in congressional races are identical in both (52% Democratic, 37% Republican), and the balance of party affiliation among registered voters is changed two points by adding in the cell phone respondents (53% Democratic, 40% Republican among the landline sample vs. 52%-41% among the combined sample).
The more serious challenge to survey research posed by cell phones is the declining absolute numbers of certain types of respondents, most notably the young. In recent Pew Research Center surveys, only about 10% of respondents in landline samples are under age 30, which is roughly half of what it should be according to the U.S. Census. Young voters reached on landlines share many of the characteristics of the cell-only group, especially in terms of political views. That is why statistical weighting of the landline samples helps to correct for the absence of the cell-only. But the shortfall of young respondents in absolute numbers means that pollsters are limited in their ability to analyze differences within this age group.
The Pew Research Center will continue to track the cell phone issue throughout the 2008 campaign, periodically polling cell phone as well as landline samples to gauge the impact of including or excluding cell phone respondents.
About the Survey
Results for this survey are based on telephone interviews conducted under the direction of Princeton Survey Research Associates International among a nationwide sample of 2,004 adults, 18 years of age or older, including an oversample of respondents ages 18-29, from June 18-29, 2008 (1,501 respondents were interviewed on a landline telephone, and 503 were interviewed on a cell phone, including 162 who had no landline telephone). Both the landline and cell phone samples were provided by Survey Sampling International.
The combined landline and cell phone data were weighted using demographic weighting parameters derived from the March 2007 Census Bureau’s Current Population Survey, along with an estimate of current patterns of telephone status in the U.S. derived from the 2007 National Health Interview Survey, using an iterative technique that simultaneously balances the distributions of all weighting parameters. The weighting procedure also accounted for the fact that respondents with both landline and cell phones had a greater probability of being included in the sample.