Sampling for Telephone Surveys
Sampling for telephone surveys is fairly simple since national lists of telephone area codes, exchanges, and even individual numbers do exist. One commonly used method begins with all the area codes in the US and then identifies all the exchanges within each area code. After this is done, a computer is programmed to dial a four-digit random sequence of numbers which is added to each combination of area code and telephone exchange. The actual number of respondents in any code is determined by the actual number of telephone numbers assigned in the geographic area for which the area code is used. This technique is called "random digit dialing." It has a major advantage over, for example using a telephone book to identify a sample-people with unlisted telephone numbers might be contacted. A major disadvantage is, however, that large numbers of telephone numbers in any area code/exchange combination might be unassigned or are business rather than home numbers. Often five calls must be made to get one working residential number.
Most survey organizations modify the pure random digit dialing technique to reduce useless phone calls. One widely used method that dramatically increases the odds of getting a residential phone number is known as the Waksberg method. This is used by the Survey Research Center at The University of Michigan for surveys conducted by telephone.