Random Digit sampling (RDD) can be accomplished through several methodologies. These are Random A, Random B, Epsem, and Cluster.
Random A methodology provides an extremely representative random digit sample. Each exchange and working block will have a probability of selection equal to its share of active working blocks. Selected telephone numbers may be protected against reuse for a period of six months. Business numbers may be eliminated.

Random B methodology provides the most efficient random digit sample available. Each exchange and working block will have a probability of selection equal to its share of listed telephone households. Numbers may be protected against reuse for a period of nine months. Business numbers may be eliminated.

Epsem (Equal Probability Selection Method) ensures that every possible telephone number – including business and protected numbers – in a working block with at least one directory-listed telephone number has an equal probability of selection. Telephone numbers are not protected. Business numbers may be identified, but not removed.

Cluster methodology is used by many pollsters and other researchers who have only one or two nights in the field for interviewing purposes. Survey questions are time-sensitive. Cluster samples are accurate and representative, without the need for callbacks or interviewing by replicate.

Using SSI’s standard RDD B methodology, a sample is drawn with a sample size equal to the number of completed interviews. For each sample element, or seed, 11-20 replacement numbers are generated within the same area code and exchange.

For more information, Contact SSI.