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 and 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.
Wireless telephone numbers also have an equal probability of selection in the ABC sample. To produce these samples, SSI begins with the latest monthly listing of every existing telephone area code and exchange. About half of these are pooled by their producers in contiguous groups of 10 100-block phone numbers, or 1,000-blocks, with information including whether each pooled 1,000-block does or does not include wireless numbers, either solely or on a shared basis with landline numbers. All such wireless-inclusive 1,000-blocks are included for sampling purposes. For numbers that are not 1,000-block pooled, wireless service information is available at the exchange level only; therefore all numbers in those exchanges also are included for sampling purposes.
All numbers used in wireless sampling are then handled at the 100-block level. Given the absence of any cell-phone directory, all 100-blocks used for sampling purposes are considered active. Shared 100-blocks that include listed landline numbers are removed from the wireless sample, because they are included in the landline frame; shared 100-blocks with no listed landline numbers are retained in the wireless frame. As such the two frames are mutually exclusive, with no overlap. (The number of cell numbers included in the landline sample for any reason – porting, forwarding, or shared 100-blocks with listed landlines – is insignificant, one-tenth of 1 percent in an SSI study. Fewer than that would be cell-only.)
Each 100-block is assigned to a county based on the billing coordinates of the exchange. The database is sorted by county code, carrier name and 100-block. A sampling interval is determined by dividing the universe of eligible 100-blocks by the desired sample size. From a random start within the first sampling interval, a systematic nth selection of 100-blocks is performed and a 2-digit random number between 00 and 99 is appended to each selected 100-block stem.