The most recent brief for the Center for Media Research gives this warning:
Phone Surveys Skewed by Cell-Only Homeowners
According to the most recent PEW research study, 12.8% of U.S. households cannot now be reached by the typical
telephone survey because they have only a cell phone and no landline telephone.
The study goes on to point out:
If people who can only be reached by cell phone were just like those with
landlines, their absence from surveys would not create a problem for polling.
But cell-only adults are very different.
The National Health Interview Survey found them to be much younger, more
likely to be African American or Hispanic, less likely to be married, and less
likely to be a homeowner than adults with landline telephones. These demographic
characteristics are correlated with a wide range of social and political
behaviors.
In early 2003, just 3.2% of households were cell-only. By the fall of 2004,
pollsters and journalists were openly worrying about the potential bias that
cell-only households might create for political surveys. The National Election
Pool's exit poll found that 7.1% of those who voted on Election Day had only a
cell phone, and these cell-only voters were somewhat more Democratic and liberal
than those who said they had a landline telephone.
Given the speed with which the number of cell-only households has increased,
there is growing concern within the polling business about how long the landline
telephone survey will remain a viable data collection tool, at least by itself.
At the annual meeting of the American Association for Public Opinion Research, a
government researcher told the audience that the size of the cell-only group
could approach 25% by the end of 2008 if the current rate of increase is
sustained.
The Pew Research Center conducted four studies that included samples of cell
phone numbe! rs as we ll as landline numbers. The surveys covered a very wide
range of topics, including use of technology, media consumption, political and
social attitudes, and electoral engagement. Comparing the cell-only respondents
with those reached on landlines allowed an assessment of the degree to which our
traditional surveys are biased by the absence of the cell-only respondents.
|
Comparisons Between Landline Samples and Cell-Only
Samples |
|
Number of survey questions compared |
46 |
|
Average (mean) difference between landline and cell-only samples across all
46 questions |
7.8% |
|
Range of differences (absolute value) |
0% - 29% |
|
Maximum change in final survey estimate when cell-only sample is blended
in |
2% |
|
Average (mean) change in final survey estimate when cell-only sample is
blended in |
0.7% |
|
Source: PEW Research Center, June 2007 |
The good news, says the report, is that none of the measures would change by
more than 2 percentage points when the cell-only respondents were blended into
the landline sample and weighted according to U.S. Census parameters on basic
demographic characteristic! s.
However, while the cell-only problem is currently not biasing polls based on
the entire population, it may very well be damaging estimates for certain
subgroups in which the use of only a cell phone is more common. According to the
most recent government estimate, more than 25% of those under age 30 use only a
cell phone. An analysis of young people ages 18-25 in one of the Pew polls found
that the exclusion of the cell-only respondents resulted in significantly lower
estimates of this age group's approval of alcohol consumption and marijuana use.
Perhaps, opines the report, excluding the cell-only respondents also yields
lower estimates of technological sophistication. For example, the overall
estimate for the proportion of 18-25 year olds using social networking sites is
57% when the cell-only sample is blended with the landline sample, while the
estimate based only on the landline sample is 50%.
Including a cell-only sample with a traditional landline-based poll is
feasible, but cell-only surveys are considerably more difficult and expensive to
conduct than landline surveys.
- Federal law prohibits the use of automated dialing devices when calling cell
phones, so each number in the cell phone sample must be dialed manually.
- It also is common practice to provide respondents with a small monetary
incentive to offset the cost of the airtime used during the interview.
- The screening necessary to reach cell-only respondents among all of those
reached on a cell phone greatly increases the effort needed to complete a given
number of interviews.
- Pew estimates that interviewing a cell-only respondent costs approximately
four to five times as much as a landline respondent.
Pollsters recognize that some type of accommodation for the cell-only
population will have to be made eventually.
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