Abt SRBI draws cell phone sample proportionate to its distribution in the country’s four U.S. Census regions, per NHIS data. Respondents are screened for cell-only status; those with landlines that they use to take calls are not interviewed by cell phone, since they are covered in the separate landline frame. Cell-only respondents’ place of residence is checked and their Census region adjusted accordingly if necessary. We currently (as of June 2014) include approximately 310 cell-only interviews in surveys of 1,000 adults.
We continue to monitor our cell-only proportions and also to evaluate potential sampling enhancements, such as the use of Census-density strata by race and cell sampling stratified by phone activity records. This summary will be updated with changes as they are implemented.
In the landline component of these surveys, a sample of landline households in the continental United States is selected by SSI via random digit dialing procedures, in which all landline telephone numbers, listed and unlisted, have an equal probability of selection.
Landline sample is drawn proportionate to its estimated distribution in the country’s nine Census divisions.
SSI starts with a database of all listed telephone numbers, updated on a four- to six-week rolling basis, 25 percent of listings at a time. This database of directory-listed numbers is then used to determine all active blocks – contiguous groups of 100 phone numbers for which more than one residential number is listed. All possible numbers in active blocks are added to the random digit database.
Until 2005, ABC News followed the industry norm of excluding all listed business numbers (compiled from sources such as Yellow Pages directories and the Dunn and Bradstreet Business Data database) from the sample. However, an ABC-led study (Merkle, Langer, Cohen, Piekarski, Benford & Lambert, 2009, Public Opinion Quarterly) found that this “cleaning” process excludes respondents who have home-based business-listed phones and no other lines at home on which they take calls, creating 3 percent noncoverage of eligible households with no offsetting gains in productivity. As a result of this evaluation, we do not exclude listed business numbers from our landline sample, with the exception of those in business-only blocks or exchanges.
Each telephone exchange in the landline sample is assigned to the county where it’s most prevalent. In the first stage of selection, the database is sorted by state and county, and the number of telephone numbers to be sampled within each county is determined using systematic sampling procedures from a random start, such that each county is assigned a sample size proportional to its share of possible numbers. In the second stage of selection, telephone numbers are sorted within county by area code, exchange and active block, and using systematic sampling procedures from a random start, individual phone numbers within each county are selected. The sampled phone numbers are pre-dialed via a non-ringing auto-dialer to reduce dialing of non-working numbers.