This study demonstrates the feasibility of cell phone RDD and confirms the need for including cell phones in RDD in order to obtain a representative population-based control group for epidemiologic case-control studies.
Our study was limited by the small number of women less than 30 years of age in the landline comparison sample, and adjustment by 5-year age group may have been insufficient to control confounding. Another limitation is that we combined data that were collected by telephone interview and self-administered Web interviews for the cell-only users, whereas the data for the landline users were collected via in-person interviews. These diverse sources may not provide comparable data, and this could account for the differences we found between cell users and landline users (14
). Some respondents may have considered the questions related to reproductive history to be sensitive. Studies have found that sensitive information and questions related to social desirability are reported more accurately on self-administered questionnaires than by telephone or in-person interviews (14
). A case-control study that compared health and residential history data collected on self-administered Web questionnaires with telephone interviews found Web questionnaires to be more reliable but found no differences between survey mode and participant characteristics for cases compared with controls (17
). We found differences by interview mode for the cell responders only for Hispanic origin, education, and exercise.
An additional limitation of this study is that information from cell phone participants reflected their status in 2008 and 2009, whereas the information from landline participants referred to their status prior to reference dates that ranged from 2005 to 2007. We believe that any changes in the characteristics of interest over this time period in adults aged 20–44 years would have been too small to produce the differences that we observed.
We found differences between the Seattle-Puget Sound area cell only and landline respondents with respect to Hispanic ethnicity, education, marital status, ever having birthed/fathered a child, obesity, and exercise. We compare our results with those from 3 population-based surveys conducted between 2005 and 2009 that collected demographic and health-related information on cell-only and landline households (3
). All 3 surveys included adults of any age and did not separate results by age and gender, which limits the comparison of results with those from our study. Only one study adjusted for sex, race, and age (3
). Our findings agree with some, but not all, of the results of these surveys (). Our study and 2 of the surveys found that cell phone-only users were less likely than landline users to have ever had a child or to be currently living with children (3
). Our study and one survey found that cell-only users were less likely to be married than were landline users (3
). The differences in both variables persisted in our data when stratified by 5-year age group (data not shown). Although there are some consistencies across studies that have compared cellular-only users with landline users, there are also several differences. These differences in demographic and lifestyle characteristics may be due to geographic variations in the distributions of these characteristics across different regions of the United States, as well as to local differences in those who are or who are not cellular-only users. Thus, the impact of choosing to include or not to include a cell phone sample for a given regional study is likely to vary on the basis of the exposures and diseases of interest, as well as on the respondent's age.
Comparison of Current Study (Seattle-Puget Sound Region, Washington State, 2005–2009) Results With Results From 3 Population-based Surveys (Entire United States, 2005 and 2009; and Georgia, New Mexico, and Pennsylvania, 2007)
Identifying and recruiting potential controls who use cellular but not landline telephones are feasible: 96% of the cell-only respondents in our study reported that their cell phones were on more than 12 hours a day, so they are accessible by cell phone. This is consistent with the 2005 finding by Tuckel and O'Neill (8
) that 94% of people with only cell phones kept them on always or most of the time. Screened individuals in our study were willing to complete brief interviews on their cellular phones or online, and more than two thirds of the men and three fourths of the women answered that they would be very or somewhat willing to complete a 90-minute in-person interview. Most of the 260 (7.5%) respondents who completed the screening online used the website information left on their voice mail and never spoke to an interviewer. We believe that the option of completing the interview online may have contributed to a higher interview response, because 37% of the interviews were completed online. A Census Bureau survey conducted in 2008 found that nearly half of cell-only households reported a preference for completing information forms online (18
). The option of self-administrated online screening and interviewing needs further exploration. Recontact of numbers that were unanswered or resulted in refusal during the initial series of calls improved response proportions; 17% of the 5,079 numbers that we recontacted were successfully screened.
An important consideration with respect to implementation of cellular telephone-based RDD is response proportions. The screening response that we experienced with cellular RDD is much lower than the ~75% screening response that we achieved for landline RDD during a similar time period but similar to the highest response (47.5%) from recent surveys targeting cell phones in other areas of the United States (7
). The lower screening proportion for cell phone RDD was driven by the relatively high proportions of screening refusals and “voice mail” on all attempts. We suspect that some of the cell phones that were always answered by voice mail belonged to people who primarily use landline telephones and thus would have been ineligible for our study. This suspicion is supported by data from a study that found a higher proportion of cell-only users kept their cell phones on all of the time than respondents with both cell and landline telephones (8
). Nevertheless, the low screening response proportions achieved can jeopardize the validity of a case-control study that uses this method.
Another important limitation of cell phone RDD is that individuals may and often do retain a cell phone number when moving into or out of a study's geographic area, with 10% of the cellular numbers screened in our study belonging to individuals who resided outside the 3-county area, but we have no way of measuring how many people resided in our 3-county area but had a cellular number assigned to another geographic area and were not in our sampling frame. One solution is to eliminate cell-only cases with cellular numbers assigned in another geographic area to achieve comparability with controls. However, sample size and representativeness of the sample may be compromised if the number of cases excluded for this reason increases.
A more difficult challenge is to integrate landline and cellular telephone samples appropriately because landline telephones are associated with households, whereas cellular telephone numbers are primarily associated with individuals (4
). One possibility is to screen cellular telephone numbers the same way landline numbers are screened by enumerating all members of the respondent's household and randomly selecting one person from all who are eligible (4
). Landline RDD reduces the probability of inclusion by one half when 2 telephone numbers ring in the same household. A similar reduction will need to account for cell phone sharing and multiple cell phones among eligible respondents within a single household. We found that 5% of cell-only users share their cell phone one third of the time or more with at least one other adult who is 20–44 years of age, but we did not ascertain whether or not the adult sharing the cell phone lived in the same household or lived in a household with a landline telephone. A 2007 study found that 10%–20% of cell-only adults reported sharing their cell phone with another adult of any age at least one third of the time, with variation by geographic area (7
A dual frame of landline and cell phone RDD with landline/cell-only status of cases as one of the frequency matching variables could be explored. Matched or stratified analyses by cellular status should be considered, particularly if the primary data collection is done by telephone, because differences of interview data have been found between landline and cellular telephone interviews (7
). Stratification will also facilitate evaluation of the impact of different screening proportions between landline and cellular RDD.
Landline RDD alone can no longer produce a representative sample of the young adult US population and must be supplemented with cellular RDD (1
). Using landline RDD to recruit controls 45–64 years of age and 65 years of age or older currently would exclude far fewer controls: 13% and 5% of the population, respectively (6
). However, these percentages continue to rise (6
). Methods to increase RDD screening response, such as refusal recontact and a monetary “thank you” to respondents, should be considered. The impact of nonresponse bias could substantially hamper scientific inferences if cellular RDD screening response proportions cannot be improved and if nonrespondents cannot be characterized. Further research is needed into methodological solutions for increasing cellular screening response, combining data from landline and cellular RDD, and accounting for geography, shared cellular telephones, and multiple cell phone households.