摘要：Objectives. This study explored differences between refusers and participants in a longitudinal study with extensive baseline and follow-up information. Methods. Results of a trial comparing 791 participants and 401 community-residing older adults who refused to participate in a study concerning preventive home visits were examined. Information was collected from interviews, insurance records, and government files. Results. Despite similarities in terms of age, sex, and self-perceived health at baseline, 3-year follow-up data indicated that refusers had a 1.58-fold higher risk of entering a nursing home than participants. There were additional differences between refusers and participants when refusers were categorized in 4 subgroups based on self-reported reason for refusal (too ill, too healthy, no interest, and other reasons). Conclusions. Future studies should include follow-up data to allow comparisons between refusers and participants and should address the presence of multiple subgroups of refusers. (Am J Public Health. 2002;92:445–450) Concerns about biases introduced by refusers in studies of the health of the elderly have been raised repeatedly. 1 In descriptive studies, refusal or nonresponse can lead to selection bias if disease or exposure status is associated with selection of subjects. 2, 3 In randomized, controlled studies, refusal can decrease the generalizability of the findings if subgroups with characteristics associated with intervention effects are misrepresented. Both consequences can have serious ramifications. In a systematic literature search, we found 13 published studies of health or health care use among older people that had as a main topic analysis of differences between refusers and participants. 4– 16 Of these 13 studies, 4 included both baseline and follow-up data, 4– 7 and 9 focused on baseline data alone. 8– 16 Several of the studies reported (mostly minor) differences in sociodemographic characteristics between refusers and participants. The most frequently reported differences were as follows: older age, 5, 10, 12, 14 lower educational level, 8, 10– 13 and higher proportion of women among refusers than among participants. 5, 7 With regard to health, health behavior, and health care use, the studies reported variable findings. Several authors reported that refusers were worse in regard to health-related status (e.g., lower levels of self-perceived health 8, 12, 15 and higher rates of mortality, 6 cognitive impairment, 6, 9, 13 health problems or risks, 6, 11, 13, 15 and health care use 15 ) than participants. On the other hand, some studies reported that refusers had a lower prevalence of health problems or risks 7, 10, 14 than participants. Overall, in terms of health status, most studies identified one subgroup of older individuals who were underrepresented in the study, and the presence of this group was usually interpreted as not having an effect on the study conclusions. Several facts may account for these diverse research findings. First, intervention intensity and type of subject involvement varied between the studies and might have affected differences between refusers and participants. For example, one study 6 showed that if individuals were asked to respond in a personal interview, there were only minor differences between refusers and participants, but if the same individuals were asked to return a mail questionnaire, refusers differed significantly from participants. Those who refused to return mail questionnaires had lower levels of cognitive functioning than participants, probably because cognitively impaired individuals have difficulties answering and returning such a questionnaire. Second, the health risk factors targeted influenced participation. For example, individuals without vision problems were more likely to refuse to participate in a study of an ophthalmologic intervention, 12 individuals without oral health problems were more likely to refuse to participate in a dental intervention program, 10 and individuals who perceived few health benefits from seeing a geriatric health specialist were more likely to refuse to participate in a geriatric assessment study. 16 Third, methodological factors might account for the differences in previous results. The studies varied in regard to the nature and completeness of the information collected. For example, none of the studies with follow-up information included combined data on demographics, health status, and health care use among refusers and participants. Thus, the diverse findings may have derived from the use of selected subsets of variables, thereby limiting the multidimensionality of the analyses. Finally, completeness of information on refusers varied. For example, only 2 studies reported that the percentage of missing health status or health care use information was lower than 20% among refusers, 5, 7 and in most studies a proportion of the original population could never be contacted, thus increasing the number of potentially eligible persons with missing information. In summary, our review of previous research on refusal to participate among older adults shows that these studies involved different methods, strategies, age groups, and interventions, making conclusions difficult. The discordant findings between the studies are partly explained by the intensity and type of intervention offered; however, there is no explanation as to why some studies show that refusers tend to be healthier than participants while others show that refusers tend to be sicker than participants. It has been suggested, but not confirmed by empirical data, that sick persons do not participate in preventive trials and that healthy persons do not participate in therapeutic trials. 17 On the basis of this information, it is quite likely that refusers are a heterogeneous group of people who refuse for different reasons and are likely to differ in varying ways depending on reasons for nonparticipation. To resolve the problems of previous studies, we analyzed the database of a recent randomized, controlled intervention trial for which we had detailed baseline and long-term follow-up information with little missing data on either participants or refusers. 18 The completeness of this database allowed us to (1) determine whether conclusions based on comparisons of baseline data between refusers and participants differ from those drawn from baseline and follow-up data combined and (2) explore factors involved in refusal to participate to improve understanding of the unexplained health differences between refusers and participants revealed in previous studies.
