Add-a-digit sampling is a method of creating a sample of telephone numbers to reach the general public within some geopolitical area of interest. This method is related to directory sampling in that the first step involves drawing a random sample of residential directory-listed telephone numbers from a telephone directory that covers the geographic area of interest. In add-a-digit sampling, the selected directory-listed telephone numbers are not called. Rather, they form the seeds for the list of numbers that will be called. For each directory-listed telephone number drawn from the telephone directory, the last digit of the telephone number is modified by adding one to the last digit. The resulting number is treated as one of the telephone numbers to be sampled. This is the simplest ...
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