Suffix Banks

Telephone numbers in the United States, Canada, and the Caribbean consist of 10 digits divided into three components: The first three digits are the area code; the next three digits are the prefix or exchange; and the final four digits are the suffix, or local number. For each area code-prefix combination, the 10,000 possible numbers for a suffix can be subdivided into banks or blocks of consecutive numbers: 1000-banks (Nnnn), 100-banks (NNnn), or 10-banks (NNNn).

As the sampling method of random-digit dialing (RDD) evolved, the underlying assumption was that residential telephone numbers tended to cluster. Based on this assumption, one of the earliest methodologies consisted of simply adding one to a directory-listed number or randomizing the last digit of a directory-listed number. Both methodologies proved to introduce bias. At the other end of the spectrum, randomizing all four digits of suffixes in residential exchanges eliminated bias but proved to be too inefficient and expensive to field.

Since the early mechanical switches came in banks of 1,000 numbers, telephone companies avoided purchasing unnecessary equipment by assigning numbers in suffix blocks of 1,000 numbers (or “1000-banks”). Later research showed that this clustering of numbers extended to 100-banks as well, primarily to accommodate businesses.

Two classes of RDD methodologies were developed to take advantage of this clustering effect: two-stage designs (such as Mitofsky-Waksberg) and single-stage, list-assigned designs that use a database of listed numbers to qualify banks (1000-banks or 100-banks) for inclusion in the telephone frame, based on the presence of directory-listed numbers. Although not all countries have fixed-length telephone numbers, these designs have been successfully adapted for use around the world.