THE EFFECTS OF ADVANCED LETTERS ON RESPONSE RATES IN LINKED TELEPHONE SURVEYS

Robert M. Groves, Universi ty of Michigan, Cecelia Snowden, National Center for Health Statistics Introduction A "linked telephone survey" (LTS) is defined as a data collection effort on persons, families, or households related to those measured in an earlier personal interview data collection from a sample. In this paper "linked telephone survey" will be used to describe a reinterview of persons measured previously in the National Health Interview Survey (NHIS). This paper describes the motivation, design, and findings of a methodological investigation into the use of advance letters for linked telephone surveys for the NHIS. 1. Alternat ive D e s i g n s for Col lect ing Information from the U.S. Househo ld Populat ion The choice of method to collect information on the U.S. household population mus t consider both errors and costs of the alternatives. Decisions must be made regarding sampling f rames (the mater ials used to identify the population) and data collection modes. The sampling frame options are typically Census address f rames methods (as used in the Current Population Survey and NHIS prior to 1985), a rea probability methods (as used in the 1985 NHIS), telephone number frames, and other address methods. The data collection methods of chief interest are the personal visit interview and the telephone interview. The relative coverage errors (failure to include some of the household population) of address f rame and area frame methods are not well documented in the survey literature. The use of the telephone frame to study the household population appears to miss between 6 and 7 percent of the persons covered by the Census address frame methods. Thornberry and Massey (1983) have documented tha t those without telephones tend to have lower family incomes, to have smaller households, and to live in rural areas. The proportion of the elderly who have telephones is relatively larger than the proportions in other groups. Data collection modes also differ in their nonresponse and measu remen t error characteristics. Groves and Kahn (1979), DeMaio (1984), O'Neil et al. (1980), Fitti et al. (1979), and Sykes and Hoinville (1"985) have all discussed problems of response rates in first time telephone surveys. In telephone surveys tha t had no prior contact with the sample household (sometimes called "cold" telephone surveys), across various organizations, over several years of research, cooperation tends to be lower than tha t obtained in comparable personal visit interviews. Cannell et al. (1987) have shown that nonresponse is especially high among the elderly in cold telephone surveys. In terms of measuremen t error, however, differences among modes are difficult to find in the survey methodological l i terature. Hochstim (1967) found fewer reports of intake of alcoholic beverages among women in personal visit surveys than in the other modes, but few other differences. Groves and Kahn (1979) similarly found few differences between personal and telephone interviews. There were, however, some tendencies for respondents to give shorter answers to open questions in the telephone mode. Cannell et al. (1987), in a adaptat ion of the NHIS in a cold telephone format, found more reporting of health events in the telephone mode than obtained by Census Bureau interviewers in the NHIS. The inference from this study is clouded by potential organizational differences tha t are confounded with mode differences. Groves and Magilavy (1986) present some evidence tha t argues that response variance associated with interviewer differences may be reduced through the centralization of interviewers that occurs in most telephone surveys. On the whole, however, there are few documented and interpretable differences in statistics obtained in cold telephone surveys and those in personal visit surveys. Of all the errors in telephone surveys, nonresponse error seems most intractable. Efforts to increase response rates on the telephone in health surveys have been largely unsuccessful (e.g., O'Neil et al., 1979). Lower telephone survey response rates among the elderly are particularly disturbing for a health survey because the elderly have distinctive health characteristics. This concern led to efforts in the use of advanced letters with a linked telephone survey to decrease nonresponse error over cold telephone surveys. 2. Advance Letters and Response Rates The hypothesized causes of improved cooperation with advance letters focus either on changes in the interviewer behavior or changes in respondent attitudes. Let ters might improve response ra tes by increasing the self-confidence of interviewers, allowing them to refer to an a t tempt to notify the sample household about their call (whether or not the letter was actually received or read). Calling attention to the letter relieves the interviewer from single-handedly legitimizing her requests for the respondents ' time. The interviewer is more fully an agent sent by an organization tha t used its resources to forewarn the respondent of the call. If this hypothesis is true a portion of the effect of the advance letter corresponds to changes in interviewer behavior given knowledge of the advance letter. This hypothesis could be tested empirically. For half of the sample receiving letters interviewers could be alerted that a letter had been sent, and directed to refer to the letter in introductory comments with the respondent. For the other half of the sample receiving letters no such knowledge would be given to the interviewers. Correspondingly, for the cases not receiving letters, half would make reference to a letter and half would not. The t r ea tment of having interviewers refer to a letter when none had been sent involves a small deception of both interviewers and respondents and needs to be discussed. It would, however, allow us to measure a pure effect of interviewer behavior, given knowledge of an advance letter. The operational complication with this plan is tha t some respondents who received the letter will refer to the letter even without the interviewer 's knowledge of the letter. The deception inherent in the experiment will thus be revealed to some interviewers during the study. Some assessment of the damage of this possibility must be made. The most striking result in the l i terature on advance letters is tha t of Cannell and Fowler (1965) tha t 44 percent of the respondents in an NHIS-like interview reported that they had not received the letter and brochure describing the survey. A total of 33 percent reported reading the letter carefully, and 16 percent quickly. Thus, over 50 percent said they had read neither the letter nor the brochure. A question raised about these results concerns the actual receipt of the letter; the authors estimate tha t about 73 percent of the letters were actually received. Among those addresses where there was strong evidence of the letter being delivered, 58 percent of the respondents reported reading the letter. There is some evidence that women read the mater ia ls more frequently than men, and that lower income groups read more frequently than higher groups. These results suggest that three features of the experiment mus t be implemented. First, there should be an experimental check to inquire of respondents whether or not they received a letter, and if so, whether they read it. Second, attention must be given to what aspects of the letter or envelope will increase the level of reading of the letter. With the low levels of reading reported in the above study, it is unlikely tha t the contents of the letter could have any effect. Third, increased personalization of the letter can be at tempted in an LTS design. In a first-time interview situation, the normal