The following commentary is based on a presentation at the Annual Meeting of the American Association for Public Opinion Research, Hollywood, Florida, May 14-17, 2009.
As the number of adults reachable only by cell phone continues to grow, more telephone surveys are including cell phone samples to ensure that their results are representative of the U.S. population. One issue of particular concern in surveys that include cell phones is the accuracy of geographic information that is derived from cell phone numbers; this information that accompanies the sample is used by many surveyors for geographic sampling and analysis. Being able to identify the location of respondents with precision is important for accurately sampling people in particular areas and for analyzing local and regional differences in respondents' attitudes and behaviors. This problem is exacerbated by the fact that the wireless-only are more geographically mobile than those with landline phones.
There are several differences in the geographic information provided with landline and cell phone numbers. Because cell phones are not wired to a particular location, the number associated with that phone does not have the same meaning geographically that it has for landline phones. The area code and exchange associated with a landline telephone number allows it to be located fairly precisely. However, there is no requirement that wireless phones numbers be associated with a particular address or even with a particular geographic area. Even if someone chooses a number "near" the area where they live or work, the service areas for wireless phones can be larger than those for landline phones so accurately locating someone is much more difficult. And, of course, people may obtain a telephone in one location and move to an entirely different location while retaining the phone number.
To assess the accuracy of the geographic information provided with wireless telephone samples, we test whether the sample information matches geographic data derived from respondents' self-reported zip code at the regional, state and county level and compare the results to those for the landline sample frame. The data for this analysis come from six general population surveys conducted in the Fall of 2008. The combined dataset includes 10,430 landline respondents and 3,460 cell respondents, including 1,160 cell phone only respondents. This design allows us to test the validity of geographic information from landline and cell phone samples simultaneously and to analyze the accuracy of the information for different types of cell respondents (cell only, cell mostly, and others who use their cell phones less frequently). We also use data from a survey focused on geographic mobility to evaluate whether types of phone-use groups differ in their patterns of mobility.
Read the full commentary Accurately Locating Where Wireless Respondents Live Requires More Than A Phone Number on the Pew Research Center's Web site.