Cell phone 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 cell phone numbers, either solely or on a shared basis with landline numbers. All such cell-inclusive 1,000-blocks are included for sampling purposes. For numbers that are not 1,000-block pooled, cell phone 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 cell phone 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 cell phone sample, because they are included in the landline frame; shared 100-blocks with no listed landline numbers are retained in the cell phone 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 one 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.
Unlike sampled landlines, sampled cell phone numbers are not pre-dialed via non-ringing auto-dialer, and are hand-dialed by Abt SRBI interviewers.
In each sample, phone numbers are released for interviewing in replicates by Census region (cell) or division (landline) to allow for sample control. Numbers are called multiple times during the field period in multi-night polls. Interviews are conducted via a computer-assisted telephone interviewing (CATI) system. Abt SRBI’s professional interviewers, and their supervisors, are extensively trained in interviewing practices, including techniques designed to achieve the highest possible respondent cooperation.
Cell-only respondents are not offered compensation. A reimbursement check is offered if use of minutes is raised as an objection and the respondent subsequently supplies his or her mailing address; on average this occurs in two or three cases per survey.
As of April 2013, Spanish-language interviewing was added to full-length ABC/Post polls for respondents who indicate a preference to be interviewed in Spanish. (Spanish-language interviewing in SSRS omnibus surveys began in October 2009.)