Wireless Substitution: Early Release of Estimates From the National Health Interview Survey, January - June 2008
by Stephen J. Blumberg, Ph.D., and Julian V. Luke, Division of Health Interview Statistics, National Center for Health Statistics
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Preliminary results from the January-June 2008 National Health Interview Survey (NHIS) indicate that the number of American homes with only wireless telephones continues to grow. More than one out of every six American homes (17.5%) had only wireless telephones during the first half of 2008, an increase of 1.7 percentage points since the second half of 2007. In addition, more than one out of every eight American homes (13.3%) received all or almost all calls on wireless telephones despite having a landline telephone in the home. This report presents the most up-to-date estimates available from the federal government concerning the size and characteristics of these populations.
This report is published as part of the NHIS Early Release Program. In May and December of each year, the Centers for Disease Control and Prevention’s National Center for Health Statistics (NCHS) releases selected estimates of telephone coverage for the civilian noninstitutionalized U.S. population based on data from NHIS, along with comparable estimates from NHIS for the previous 3 years. The estimates are based on in-person interviews. NHIS interviews are conducted continuously throughout the year to collect information on health status, health-related behaviors, and health care utilization. The survey also includes information about household telephones and whether anyone in the household has a wireless telephone (also known as a cellular telephone, cell phone, or mobile phone).
Two additional reports are published as part of the Early Release Program. Early Release of Selected Estimates Based on Data from the National Health Interview Survey is published quarterly and provides estimates of 15 selected measures of health. Health Insurance Coverage: Early Release of Estimates from the National Health Interview Survey is also published quarterly and provides additional estimates of health insurance coverage.
For many years, NHIS has included questions on residential telephone numbers to permit recontact of survey participants. Starting in 2003, additional questions determined whether the family’s telephone number was a landline telephone. All survey respondents were also asked whether “you or anyone in your family has a working cellular telephone.” A family can be an individual or a group of two or more related persons living together in the same housing unit. Thus, a family can consist of only one person, and more than one family can live in a household (including, for example, a household where there are multiple single-person families, as when unrelated roommates are living together).
In this report, families are identified as wireless families if anyone in the family had a working cellular telephone. This person (or persons) could be a civilian adult, a member of the military, or a child. Households are identified as wireless-only if they include at least one wireless family and if there are no working landline telephones inside the household. Persons are identified as wireless-only if they live in a wireless-only household. A similar approach is used to identify adults living in households with no telephone service (neither wireless nor landline). Household telephone status (rather than family telephone status) is used in this report because most telephone surveys draw samples of households rather than families.
From January through June 2008, household telephone status information was obtained for 16,070 households that included at least one civilian adult or child. These households included 30,150 civilian adults aged 18 years and over and 11,238 children less than 18 years of age. Analyses of demographic characteristics are based on data from the NHIS Family file. Demographic data for all civilian adults living in interviewed households were used in these analyses. Estimates stratified by poverty are based only on reported income. Household income was unknown for nearly 18% of adults.
Analyses of selected health measures are based on data from the NHIS Sample Adult file. Health-related data for one civilian adult randomly selected from each family were used in these analyses. From January through June 2008, data on household telephone status and selected health measures were collected from 12,594 randomly selected adults.
Because NHIS is conducted throughout the year and the sample is designed to yield a nationally representative sample each week, data can be analyzed quarterly. Weights are created for each calendar quarter of the NHIS sample. NHIS data weighting procedures have been described in more detail in an NCHS published report (Series Report Number 2, Volume 130 pdf icon[PDF – 300 KB]). The estimates using the January-June 2008 data are being released prior to final data editing and final weighting to provide access to the most recent information from NHIS. The resulting estimates should be considered preliminary and may differ slightly from estimates using the final data files.
Point estimates and 95% confidence intervals were calculated using SUDAAN software to account for the complex sample design of NHIS. Differences between percentages were evaluated using two-sided significance tests at the 0.05 level. Terms such as “more likely” and “less likely” indicate a statistically significant difference. Lack of comments regarding the difference between any two estimates does not necessarily mean that the difference was tested and found to be not significant. Due to small sample sizes, estimates based on less than 1 year of data may have large variances, and caution should be used in interpreting these estimates.
From 2003 to 2006, families were considered to have landline telephone service if the survey respondent provided a telephone number, identified it as “the family’s phone number,” and said that it was not a cellular telephone number. If the family’s phone number was reported to be a cellular telephone number, the respondent was asked if there was “at least one phone inside your home that is currently working and is not a cell phone.”
In 2007, the questionnaire was changed so that all survey respondents were asked if there was “at least one phone inside your home that is currently working and is not a cell phone,” unless the respondent indicated not having any phone when asked for a telephone number.
From 2003 to 2006, the questions about cellular telephones were asked at the end of the survey. Because of incomplete interviews, more than 10% of households were not asked about wireless telephones. In 2007, these questions were moved earlier in the survey, resulting in fewer families with unknown wireless telephone status.
In 2007, a new question was added to the survey for persons living in families with both landline and cellular telephones. Respondents were asked to consider all of the telephone calls that their family receives and to report whether “all or almost all calls are received on cell phones, some are received on cell phones and some on regular phones, or very few or none are received on cell phones.” This new question permits the identification of persons living in “wireless-mostly” households, defined as households with both landline and cellular telephones in which all families receive all or almost all calls on cell phones.
Finally, in 2007, the questionnaire was redesigned to improve the collection of income information. Initial evaluations suggest that poverty estimates are generally comparable to years 2006 and earlier. However, as a result of the changes, the poverty ratio variable has fewer missing values in 2007 compared with prior years.
In the first 6 months of 2008, more than one out of every six households (17.5%) did not have a landline telephone, but did have at least one wireless telephone (Table 1). Approximately 16.1% of all adults–more than 36 million adults–lived in households with only wireless telephones; 17.0% of all children–more than 12 million children–lived in households with only wireless telephones.
The percentage of adults living in wireless-only households has been steadily increasing (see Figure). As noted, during the first 6 months of 2008, nearly one out of every six adults lived in wireless-only households. One year before that (that is, during the first 6 months of 2007), one out of every eight adults lived in wireless-only households. And 2 years before that (that is, during the first 6 months of 2005), only 1 out of every 15 adults lived in wireless-only households.
The percentage of adults and the percentage of children living without any telephone service have remained relatively unchanged over the past 3 years. Approximately 2.5% of households had no telephone service (neither wireless nor landline). Nearly 5 million adults (2.1%) and 2 million children (2.5%) lived in these households.
The percentage of U.S. civilian noninstitutionalized adults living in wireless-only households is shown by selected demographic characteristics and by survey time period in Table 2. For the period January through June 2008:
- Nearly two-thirds of all adults living only with unrelated adult roommates (63.1%) were in households with only wireless telephones. This is the highest prevalence rate among the population subgroups examined.
- One-third of adults renting their home (33.6%) had only wireless telephones. Adults renting their home were more likely than adults owning their home (9.0%) to be living in households with only wireless telephones.
- More than one in three adults aged 25-29 years (35.7%) lived in households with only wireless telephones. Approximately 31% of adults aged 18-24 years lived in households with only wireless telephones.
- As age increased from 30 years, the percentage of adults living in households with only wireless telephones decreased: 19.1% for adults aged 30-44 years; 9.2% for adults aged 45-64 years; and 2.8% for adults aged 65 years and over.
- Men (18.0%) were more likely than women (14.4%) to be living in households with only wireless telephones.
- Adults living in poverty (26.0%) and adults living near poverty (22.6%) were more likely than higher income adults (14.2%) to be living in households with only wireless telephones.
- Adults living in the South (19.6%) and Midwest (17.8%) were more likely than adults living in the Northeast (9.8%) or West (13.7%) to be living in households with only wireless telephones.
- Non-Hispanic white adults (14.6%) were less likely than Hispanic adults (21.6%) or non-Hispanic black adults (18.5%) to be living in households with only wireless telephones.
Among households with both landline and cellular telephones, 22.7% received all or almost all calls on the cellular telephones, based on data for the period January through June 2008. These wireless-mostly households make up 13.3% of all households. These estimates are statistically similar to the estimates from the last 6 months of 2007.
Approximately 32 million adults (14.4%) lived in wireless-mostly households during the first 6 months of 2008. Table 3 presents the percentage of adults living in wireless-mostly households by selected demographic characteristics and by survey time period. For the period January through June 2008:
- Adults with college degrees (17.1%) were more likely to be living in wireless-mostly households than were high school graduates (12.5%) or adults with less education (10.0%).
- Adults living with children (18.1%) were more likely than adults living alone (10.1%) or with only adult relatives (12.8%) to be living in wireless-mostly households.
- Adults living in poverty (10.8%) and adults living near poverty (10.3%) were less likely than higher income adults (17.1%) to be living in wireless-mostly households.
- Adults living in metropolitan areas (15.0%) were more likely to be living in wireless-mostly households than were adults living in more rural areas (12.1%).
Most major survey research organizations, including NCHS, do not include wireless telephone numbers when conducting random-digit-dial telephone surveys. Therefore, the inability to reach households with only wireless telephones (or with no telephone service) has potential implications for results from health surveys, political polls, and other research conducted using random-digit-dial telephone surveys. Coverage bias may exist if there are differences between persons with and without landline telephones for the substantive variables of interest.
The NHIS Early Release program updates and releases estimates for 15 key adult health indicators every 3 months. Table 4 presents estimates by household telephone status (landline, wireless-only, or without any telephone service) for all but two of these measures. (“Pneumococcal vaccination” and “personal care needs” were not included because these indicators are limited to adults aged 65 years and over.) For the period January through June 2008:
- The prevalence of binge drinking (i.e., having five or more alcoholic drinks in 1 day during the past year) among wireless-only adults (37.7%) was twice as high as the prevalence among adults living in landline households (17.2%). Wireless-only adults were also more likely to be current smokers.
- Compared with adults living in landline households, wireless-only adults were more likely to report that their health status was excellent or very good, they were more likely to engage in regular leisure-time physical activity, and they were less likely to have ever been diagnosed with diabetes.
- The percentage without health insurance coverage at the time of the interview among wireless-only adults (28.3%) was twice as high as the percentage among adults living in landline households (13.6%).
- Compared with adults living in landline households, wireless-only adults were more likely to have experienced financial barriers to obtaining needed health care, and they were less likely to have a usual place to go for medical care. Wireless-only adults were also less likely to have received an influenza vaccination during the previous year.
- Wireless-only adults (49.3%) were more likely than adults living in landline households (35.8%) to have ever been tested for HIV, the virus that causes AIDS.
The potential for bias due to undercoverage remains a real and growing threat to surveys conducted only on landline telephones. For more information about the potential implications for health surveys based on landline telephone interviews, see:
- Blumberg SJ, Luke JV. Coverage bias in traditional telephone surveys of low-income and young adults. Public Opinion Quarterly 71:734-49. 2007.
- Blumberg SJ, Luke JV, Cynamon ML. Telephone coverage and health survey estimates: Evaluating the need for concern about wireless substitution. American Journal of Public Health 96:926-31. 2006.
- Blumberg SJ, Luke JV, Cynamon ML, Frankel MR. Recent trends in household telephone coverage in the United States. In JM Lepkowski et al. (eds.), Advances in Telephone Survey Methodology (pp. 56-86). New York: John Wiley and Sons, Inc. 2008.
For more information about the National Health Interview Survey or the Early Release program, or to find other Early Release reports, please see the following websites:
Blumberg SJ, Luke JV. Wireless substi-tution: Early release of estimates from the National Health Interview Survey, January-June 2008. National Center for Health Statistics. Available from: /nchs/nhis.htm. December 17, 2008.image icon
- Table 1. Percent distribution of household telephone status, by date of interview, for households, adults, and children: United States, January 2005-June 2008
- Table 2. Percentage of adults living in wireless-only households, by selected demographic characteristics and by calendar half-years: United States, January 2005-June 2008
- Table 3. Percentage of adults living in landline households with wireless telephones, by proportion of calls received on wireless telephones, by selected demographic characteristics, and by calendar half-years: United States, July 2007-June 2008
- Table 4. Prevalence rates (and 95% confidence intervals) for selected measures of health-related behaviors, health status, health care service use, and health care access for adults 18 years of age and over, by household telephone status: United States, January-June 2008