Centralization of telephone interviewing also makes it feasible for research organizations to provide interviewers with CRT displays and computer control of questionnaires. The computer brings the questions on the CRT display in the proper sequence, even skipping questions when called for by the answers to previous questions. This reduces variation and error in presentation by the interviewers. The latter can enter the respondent’s answer directly into the computer via a typewriter like terminal. This greatly reduces the time and error involved in collecting, editing, coding, and tabulating results as compared to mail and personal interviews. The computerization process permits customizing and personalizing each interview, which improves respondent rapport and quality of response. For example, at one point in the interview the respondent may report that she usually shops at the Jewel Food Store. Later the questionnaire may return to ask about the store and the computer will put the name Jewel Food in the question as it appears on the CRT display for the interviewer to read.
Figures show a series of questions taken from a telephone survey using a CRT terminal. The different segments show how the computer automatically displays the correct next questions on the CRT.
Telephone and personal interviews have another important advantage they can be used to collect information on events at the time they are happening, thus reducing the errors resulting from failure of memory. Radio and television listening rate often checked in this manner. Telephone interviews are particularly good in this respect, because the speed with they can be conducted permits a relatively large number of interviews during a short period –for example, the period during which a particular television program is on.
Control of Sample: On the surface it appears that telephone interviewing is ideally adapted to the selection of random samples. Lists of the population are available in telephone books, and a routine procedure can be established for selecting names from these lists. This advantage of the telephone method does tend to exist, but the problem is more complex. About 4.5 million households do not have telephones. The proportion with telephones is steadily expanding, and now 95 percent of all homes have telephones. Ownership varies among income groups. Virtually all families with incomes over $ 25,000 have phones, but only 80 percent of those with family incomes below $4,000, in the South and in rural areas, have phones most marketing research studies the loss of those without telephones is probably not important but for some special the home without telephones cold be significant.
A greater problem than non-telephone homes is the growing number of homes with unlisted telephone numbers. Nationally, about 20 percent of all households with telephones have unlisted numbers, but this figure becomes 30 percent in major metropolitan areas. In Chicago it is estimated that 33 percent of the residential telephones are unlisted, and another 13 percent are directory assistance only. Thus, 46 percent of the residential phones in Chicago are not in the phone book. In addition to being high in large cities, the incidence of unlisted phone numbers is twice as high among nonwhites as whites and higher in households with three or more persons and in households with one or more children.
This large number of unlisted telephones would probably have limited the growth in collection of data by telephone were it not for the development of a method of selecting phone numbers randomly whether they are listed or not. This procedure, called Random Digit Dialing (RDD), is based on the fact that all telephone numbers are a subset of all possible 10 digit numbers (area code – 3 digits; exchange – 3 digits; number – 4 digits). A computer can be used to generate a series of random 10 digit numbers, a process that will give every phone number, listed or unlisted an equal chance of being selected.