Wireless Substitution: Early Release of Estimates From the National Health Interview Survey, July-December 2009

by Stephen J. Blumberg, Ph.D., and Julian V. Luke, Division of Health Interview Statistics, National Center for Health Statistics

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Overview

Preliminary results from the July-December 2009 National Health Interview Survey (NHIS) indicate that the number of American homes with only wireless telephones continues to grow. One of every four American homes (24.5%) had only wireless telephones (also known as cellular telephones, cell phones, or mobile phones) during the last half of 2009-an increase of 1.8 percentage points since the first half of 2009. In addition, one of every seven American homes (14.9%) had a landline yet received all or almost all calls on wireless telephones. This report presents the most up-to-date estimates available from the federal government concerning the size and characteristics of these populations.

NHIS Early Release Program

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 that NHIS conducts 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.

Two additional reports are published as part of the NHIS Early Release Program. Early Release of Selected Estimates Based on Data From the National Health Interview Survey is published quarterly and provides estimates for 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 regarding health insurance coverage.

Methods

For many years, NHIS has included questions on residential telephone numbers, to permit recontacting of survey participants. Starting in 2003, additional questions were asked to determine whether a family’s telephone number reached a landline telephone. 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 (a “household”). 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 at the time of interview. 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 July through December 2009, information on household telephone status was obtained for 21,375 households that included at least one civilian adult or child. These households included 40,619 civilian adults aged 18 years and over and 14,984 children under age 18.

Analyses of demographic characteristics are based on data from the NHIS Person and Household files. Demographic data for all civilian adults living in interviewed households were used in these analyses. Estimates stratified by poverty status are based on reported income only, because imputed income values are not available until a few months after the annual release of NHIS microdata. Household income was unknown for 12% 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 July through December 2009, data on household telephone status and selected health measures were collected from 17,539 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 are described in more detail in a previous NCHS report (Vital and Health Statistics, series 2, no 130 [PDF – 300 KB]). To provide access to the most recent information from NHIS, estimates using the July-December 2009 data are being released prior to final data editing and final weighting. These estimates should be considered preliminary. If estimates are produced using the final data files, the estimates may differ slightly from those presented here.

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 by 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 comment regarding the difference between any two estimates does not necessarily mean that the difference was tested and found to be not significant. Because of small sample sizes, estimates based on less than 1 year of data may have large variances, and caution should be used in interpreting such estimates.

Questionnaire Changes in 2007

From 2003 to 2006, NHIS 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 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 the survey respondent for each family was asked if there was “at least one phone inside your home that is currently working and is not a cell phone” (unless the respondent had 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, the questions were asked 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. The respondent for the family was asked to consider all of the telephone calls his or her 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 the resulting poverty estimates are generally comparable with those from years 2006 and earlier. However, as a result of the changes, the poverty ratio variable has had fewer missing values since 2007 compared with prior years.

Telephone Status

In the last 6 months of 2009, one of every four households (24.5%) did not have a landline telephone but did have at least one wireless telephone (Table 1). Approximately 22.9% of all adults (approximately 52 million adults) lived in households with only wireless telephones; 25.9% of all children (more than 19 million children) lived in households with only wireless telephones.

The percentage of households that are wireless-only has been steadily increasing. The 4.3-percentage-point increase from the last 6 months of 2008 through the last 6 months of 2009 is nearly equivalent to the 4.4-percentage-point increase observed from the last 6 months of 2007 through the last 6 months of 2008.

The percentage of adults living in wireless-only households has also been increasing steadily (see Figure 1). During the last 6 months of 2009, more than two of every nine adults lived in wireless-only households. One year before that (i.e., during the last 6 months of 2008), 2 of every 11 adults lived in wireless-only households. And 2 years before that (i.e., during the last 6 months of 2006), only 2 of every 17 adults lived in wireless-only households.

The percentage of children living in wireless-only households is also growing. In fact, for this population, the 4.6-percentage-point increase from the first 6 months of 2009 is the largest 6-month increase observed since 2003, when NHIS began collecting data on children living in wireless-only households.

The percentages of adults and children living without any telephone service have remained relatively unchanged over the past 3 years. Approximately 2.0% of households had no telephone service (neither wireless nor landline). Nearly 4 million adults (1.7%) and 1.4 million children (1.9%) lived in these households.

Figure 1

Demographic Differences

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 July through December 2009,

More than three in five adults living only with unrelated adult roommates (62.9%) were in households with only wireless telephones. This is the highest prevalence rate among the population subgroups examined.
More than two in five adults renting their home (43.1%) had only wireless telephones. Adults renting their home were more likely than adults owning their home (14.0%) to be living in households with only wireless telephones.
Nearly half of adults aged 25-29 years (48.6%) lived in households with only wireless telephones. More than one-third of adults aged 18-24 or 30-34 (37.8% and 37.2%, respectively) lived in households with only wireless telephones.
As age increased from 35 years, the percentage of adults living in households with only wireless telephones decreased: 23.9% for adults aged 35-44; 14.9% for adults aged 45-64; and 5.2% for adults aged 65 and over. However, as shown in Table 2 and Figure 2, the percentage of wireless-only adults within each age group has increased over time.
Men (24.5%) were more likely than women (21.3%) to be living in households with only wireless telephones.
Adults living in poverty (36.3%) and adults living near poverty (29.0%) were more likely than higher income adults (19.6%) to be living in households with only wireless telephones.
Adults living in the Midwest (25.6%), South (25.4%), and West (22.2%) were more likely than adults living in the Northeast (15.1%) to be living in households with only wireless telephones.
Hispanic adults (30.4%) were more likely than non-Hispanic white adults (21.0%) or non-Hispanic black adults (25.0%) to be living in households with only wireless telephones.

Figure 2

Demographic Distributions

The demographic differences noted in the previous section are based on the distribution of household telephone status within each demographic group. When examining the population of wireless-only adults, some readers may instead wish to consider the distribution of various demographic characteristics within the wireless-only adult population. For example, although young adults aged 18-29 years were more likely than older adults to live in households with only wireless telephones, these young adults made up only 40.8% of all wireless-only adults. Young adults were a minority among all wireless-only adults because young adults made up only 22% of the total adult population.

Table 3 presents the percent distribution of selected demographic characteristics for adults living in households with only wireless telephones, by survey time period. The estimates in this table reveal that the distributions of selected demographic characteristics changed little over the 4-year period shown. The exceptions were related to sex, age, employment status, and household structure. From 2006 to the last 6 months of 2009,

The proportion of women among all wireless-only adults increased from approximately 46% to 48.2%.
Among all wireless-only adults, the proportion of adults aged 30 years and over has steadily increased. In the last 6 months of 2009, the majority of wireless-only adults (59.2%) were aged 30 and over, up from 48.4% in the first 6 months of 2006.
The proportion of employed adults among all wireless-only adults has decreased from 78.6% to 69.1%. Over the same time period, the proportion of adults with an employment status other than working, keeping house, or going to school increased. These adults (largely unemployed or retired) made up 20.2% of wireless-only adults in the last 6 months of 2009, up from 10.3% in the first 6 months of 2006.
Among all wireless-only adults, the proportion of adults living with children has steadily increased. In the last 6 months of 2009, 40.0% of wireless-only adults were living with children, up from 34.6% in the first 6 months of 2006.

Selected Health Measures by Household Telephone Status

Many health surveys, political polls, and other research are conducted using random-digit-dial telephone surveys. Until recently, these surveys did not include wireless telephone numbers in their samples. Now, despite operational challenges, most major survey research organizations are including wireless telephone numbers when conducting random-digit-dial telephone surveys. If they did not, the exclusion of households with only wireless telephones (along with the small proportion of households that have no telephone service) could bias results. This bias-known as coverage bias-could 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 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 older adults aged 65 years and over.) For the period July through December 2009,

The prevalence of binge drinking (i.e., having five or more alcoholic drinks in 1 day during the past year) among wireless-only adults (34.5%) was nearly twice as high as the prevalence among adults living in landline households (18.7%). Wireless-only adults were also more likely to be current smokers than were adults living in landline households.
Compared with adults living in landline households, wireless-only adults were more likely to report that their health status was excellent or very good, more likely to experience serious psychological distress, and less likely to have ever been diagnosed with diabetes.
The percentage without health insurance coverage at the time of interview among wireless-only adults under 65 years of age (29.2%) was more than twice as high as the percentage among adults in that age group living in landline households (13.8%).
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 (50.6%) were more likely than adults living in landline households (36.1%) to have ever been tested for human immunodeficiency virus (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. Telephone surveys limited to landline households may still be viable for health surveys of all adults and for surveys of most subpopulations regarding their health status (see American Journal of Public Health article by Blumberg and Luke, 2009). However, for health-related behaviors, health care service use indicators, and health care access measures (such as those in Table 4 ), caution is warranted when using landline surveys to draw inferences about subpopulations more likely to be wireless-only (such as young or low-income adults).

Wireless-mostly Households

The potential for bias due to undercoverage is not the only threat to surveys conducted on landline telephones. Researchers are also concerned that some people living in households with landlines cannot be reached on those landlines because they rely on wireless telephones for all or almost all of their calls. Among households with both landline and wireless telephones, 25.7% received all or almost all calls on the wireless telephones, based on data for the period July through December 2009. These wireless-mostly households make up 14.9% of all households.

The percentage of adults living in wireless-mostly households has been increasing (see Table 5). During the last 6 months of 2009, approximately 37 million adults (16.3%) lived in wireless-mostly households. This prevalence estimate was not different from the estimate for the first 6 months of 2009 (16.2%), but it was significantly greater than the estimate for the first 6 months of 2008 (14.4%).

Table 5 presents the percentage of adults living in wireless-mostly households, by selected demographic characteristics and by survey time period. For the period July through December 2009,

Adults working at a job or business (19.7%) and adults going to school (21.7%) were more likely to be living in wireless-mostly households than were adults keeping house (15.1%) or with another employment status such as retired or unemployed (9.0%).
Adults with college degrees (19.7%) were more likely to be living in wireless-mostly households than were high school graduates (14.2%) or adults with less education (11.5%).
Adults living with children (20.2%) were more likely than adults living alone (10.6%) or with only adult relatives (15.0%) to be living in wireless-mostly households.
Adults living in poverty (10.0%) and adults living near poverty (12.7%) were less likely than higher income adults (19.2%) to be living in wireless-mostly households.
Adults living in metropolitan areas (16.8%) were more likely to be living in wireless-mostly households than were adults living in more rural areas (14.5%).

Recent research by Boyle, Lewis, and Tefft (in the December 2009 issue of Survey Practice) suggests that the majority of adults living in wireless-mostly households are reachable using their landline telephone number. NHIS data cannot be used to estimate the proportion of wireless-only adults who are unreachable or to estimate the potential for bias due to their exclusion from landline surveys.

For More Information

For more information about the potential implications for health surveys that are based on landline telephone interviews, see

Blumberg SJ, Luke JV. Reevaluating the need for concern regarding noncoverage bias in landline surveys. Am J Public Health 99:1806-10. 2009.
Blumberg SJ, Luke JV, Cynamon ML, Frankel MR. Recent trends in household telephone coverage in the United States. In: Lepkowski JM et al., eds, Advances in telephone survey methodology. New York: John Wiley and Sons. pp 56-86. 2008.

The potential for bias may differ from one state to another because the prevalence of wireless-only households varies substantially across states. For more information about state-level prevalence estimates from the 2007 NHIS, see

Blumberg SJ, Luke JV, Davidson G, et al. Wireless substitution: State-level estimates from the National Health Interview Survey, January-December 2007 [PDF – 585 KB]. National health statistics report; no 14. Hyattsville, MD: National Center for Health Statistics. 2009.

For more information about NHIS and the NHIS Early Release Program, or to find other Early Release reports, please visit the following websites:

Suggested Citation

Blumberg SJ, Luke JV. Wireless substitution: Early release of estimates from the National Health Interview Survey, July-December 2009. National Center for Health Statistics. May 2010. Available from: /nchs/nhis.htm.