RANDOM DIGIT DIALING (RDD) A popular technique used to draw a sample for a telephone poll. Random digit dialing was developed because telephone directories are often out of date or too incomplete to use as the sampling frame ( Glasser and Metzger, 1972:59-64). Almost 95 percent of American homes have phones, but about 30 percent of these have unlisted numbers. Moreover, about one in five directory listings are out of date because people have moved. Yet another problem with telephone directories is that some homes have multiple phones ( M. L. Young, 1987: 100-101).
The basic version of RDD uses a computerized "random number generator" to list the telephone numbers that will be called. The virtue of this approach is that it is simple, and every working number has an equal probability of selection (EPSEM). The great drawback, however, is that up to 90 percent of the numbers called will be commercial or nonworking. A more efficient approach combines RDD with directory sampling. The add-a-digit dialing method (ADD) achieves this: an actual telephone number is drawn from the directory, then the last digit is replaced with a randomly selected number. Versions of ADD produce about 50 percent working telephone numbers ( Frey, 1983:67-68).
One other variation of RDD is the Waksberg technique. It exploits the common practice of assigning telephone numbers in blocks of one hundred. For example, if the number 523-6931 was dialed and found to be working, probably the series from 6901 to 7000 has been assigned to other telephone subscribers. Interviewers then simply call random numbers only within those blocks of one hundred that have at least one working number. Initial working numbers could be screened by random calling or by consulting telephone directory listings ( Waksberg, 1978:40-46).
Random digit dialing is not fool proof. Two problems in particular are cited: first, RDD misses people who live in houses without telephones--only about 5 percent of the national population, but a much higher proportion of poor and minority groups. Second, RDD produces too many calls to nonworking or