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Telephone numbers in the United States consist of 10 digits: The first three are the area code; the next three are the prefix or exchange; the final four digits are the suffix or local number. The 10,000 possible numbers for a suffix can be subdivided into banks of consecutive numbers: 1,000-banks (Nnnn); 100-banks (NNnn); or 10-banks (NNNn). Zero-number banks, or zero-listed banks, are banks that do not contain directory-listed residential numbers. Although including zero-number banks in a random-digit dialing frame allows for 100% coverage of landline residential telephone numbers, their inclusion can substantially reduce sample efficiency.

Based on regular analyses conducted by Survey Sampling International, only 29% of 100-banks in POTS (Plain Old Telephone Service) prefixes, and other prefixes shared by different types of service, have ...

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