Currently, almost one-in-four Americans have only a cell phone and another 18% get most of their calls on a cell phone. Because many people can no longer be reached by landline telephone, the representativeness of telephone surveys based only on a random sample of households with landline telephone service has come under increased scrutiny. Many pollsters and survey methodologists, including those at the Pew Research Center, are studying how cell phones impact telephone surveying. Public Opinion Quarterly dedicated a special issue to the topic of cell phones in 2007 Cell Phone Numbers and Telephone Surveying in the U.S. The Pew Research Center for the People & the Press began routinely including a cell phone sample in nearly all of its surveys in 2008.

Growth in the Cell-Only Population

One of the main challenges of surveying cell phone users is drawing a representative sample of this group. Drawing samples for all telephone surveys is now more complicated because of the introduction of cell phone numbers and number portability (i.e., where people can keep their numbers when they move or change service providers and can port a landline number to a cell phone). Telephone numbers are assigned different prefixes, which can be used to identify whether the number is for a landline or cell phone, but there are also mixed or shared prefixes that include both landline and cell numbers. In addition, people who forward their calls (e.g., from their landline number at home or work to their cell) may appear as a landline number even when they are actually talking on their cell phone.

Most telephone surveys use the household as the sampling unit because landline telephone numbers have typically been shared among all members living in a household. Once a sample of landline telephone numbers is drawn, a separate selection procedure is used to give all adults living in a given household a chance of selection (such as asking for the youngest adult male or female).

However, the situation is more complicated for cell phone users because cell phones are often considered individual rather than shared devices, so the person who answers the phone usually becomes the respondent, whether he or she is the primary user of the phone or shares the cell phone with others. Although some surveyors have experimented with selecting among the users of a shared cell phone, it is still uncertain whether the benefits of this approach outweigh the disadvantages, such as potentially lower response rates. In addition, many people under the age of 18 (and thus not eligible for most national surveys) have cell phones. Substantial time and costs are incurred screening out these ineligible respondents.

Several additional issues arise when identifying the geographic location of a cell phone number. The geographic information that can be derived from cell phone numbers is not as precise as it is for landline telephone numbers. The boundaries of wireless service areas are often larger than landline service areas. The geographic information is based on the rate center where the phone was purchased, rather than where the person lives. And many people move without changing their cell phone numbers.

Based on a comparison of geographic information provided with the sample to that derived from respondents’ self-reported zip codes, as many as 10% of cell phone respondents live in a different state and nearly 40% in a different county than was indicated by the sample. This issue is of particular concern for sampling cell phones within a geographic area. Although respondents who do not live in the area may be identified by a screener question, people who do live in the area – but have cell phones from elsewhere – are likely to be excluded from the survey. In addition, although estimates of the proportion cell only are now available for most states, it is still unclear how reliable these estimates are. Further, cell phone penetration rates are still not available for smaller geographic areas, such as counties. Because of this, it is difficult to accurately sample cell phone numbers and select cell phone numbers proportional to their size within these geographic areas.

In addition to the different procedures necessary for sampling cell phone numbers, there are also substantial challenges with interviewing people on their cell phones. Challenges that arise in conducing cell phone interviews are discussed in more detail in Cell phone surveys.

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