Sampling for Telephone Surveys

Sampling for telephone surveys is fairly simple as national lists of telephone area codes, exchanges, and even individual numbers 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 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 (RDD). It has advantages over using a telephone book to identify a sample — people with unlisted telephone numbers might be contacted, and there is no telephone book listing of cell phone numbers. A disadvantage is, however, that large numbers of telephone numbers in any area code/exchange combination might be unassigned or are business numbers rather than home numbers. Often five calls must be made to get one working residential number.

RDD can be used to include cell phones as well as land lines in the sample. Many survey houses, however, do not call cell phones for a variety of reasons — many people do not want cell phone calls from unidentified callers; the cell phone owner may be driving, and so on. Cell phone-only households are on the rise in the US, so the problem of contacting people for surveys over the phone is growing. As of 2011, nearly one-third of US households (31.6 percent) were cell phone-only. Much higher percentages of younger people and poorer people lived in this type of household. Some 58 percent of those aged 18 to 24 and nearly half (46.8 percent) of people living in poverty live in cell phone only households. Cell phone-only households also varied widely by state — from 15 percent in Rhode Island to nearly 45 percent in Idaho.

Most survey organizations modify the pure random digit dialing technique to reduce the number of useless phone calls. One widely used method that dramatically increases the odds of getting a working residential phone number is the Waksberg method.