Mitofsky-Waksberg Sampling

Mitofsky-Waksberg sampling is a two-stage, clustered approach for selecting a random sample of telephone numbers. Developed by Warren Mitofsky and Joseph Waksberg in the 1970s, this was an innovative approach designed to improve the operational efficiency of telephone samples through reductions in the proportion of unproductive numbers dialed. Prior to the development of Mitofsky-Waksberg sampling, unrestricted random-digit dial (RDD) was used, but this method was operationally inefficient as it led interviewers to call far too many nonworking numbers. Mitofsky-Waksberg sampling (including modified versions of the basic approach) was the predominant approach used for selecting samples for RDD telephone surveys throughout the 1970s and 1980s, but it was largely supplanted by list-assisted RDD by the early 1990s.

An understanding of the various approaches used for RDD sampling requires some knowledge of the structure of a telephone number. In the United States, telephone numbers are 10-digit strings. The first three digits are the area code, and the first six digits are the telephone exchange. A 100-bank is a set of telephone numbers having the same first eight digits. Historically, telephone numbers were geographically clustered. However, under provisions of the Telecommunications Act of 1996, customers are able to retain their telephone numbers when switching from one telephone service provider to another, even when that switch involves a geographic move or a switch between landline service and cellular service. This is called “porting” a telephone number.

Telephone exchanges are designated for particular uses (e.g. cellular only, plain old telephone service [POTS] only, cellular and paging). For RDD surveys, the sampling frame of telephone exchanges has typically been based on those exchanges that are designated for POTS (i.e. landline) use. However, within that subset of exchanges, not every number is assigned, and not every assigned number is residential. Thus, efficiency gains may be achieved by reducing the number of unproductive (nonworking or nonresidential) telephone numbers that are dialed.