Participants and procedure
We used data obtained from the National Survey of the Japanese Elderly (NSJE), a nationally representative survey of older Japanese aged 60 and above. The NSJE started in 1987, and the participants were interviewed every three to four years until 2006. In 1999, people aged 70 and above were recruited shortly before the long-term care insurance system was introduced in 2000, and 1,635 people participated in the baseline survey (response rate = 81.8%). The participants were followed up in 2002 and 2006. We thus used the seven-year three-wave longitudinal data for the subsequent analysis. The detailed methodology, including the research design and the response rates, is available on the website of the NSJE (JAHEAD/NSJE Project Group, n.d.).
The sampling procedure is illustrated in Fig. 1. The selection criteria were (i) 70 years old and above; (ii) self-report data provided at the first survey; and (iii) living at home at baseline. The exclusion criteria were (iv) participating in the first survey but lost to follow-up and (v) admission to a long-term care facility or died in 1999 just after the first participation. As a result, 1,290 respondents were selected for this study.
Institutionalization
The outcome was measured as self- or proxy-reported years of institutionalization. In this study, we created a composite outcome by defining institutionalization as admission to long-term care facilities enacted by the long-term care insurance and long-term hospitalization. Then, time to institutionalization was calculated as the year of the baseline survey (1999) to the year of the last follow-up survey (2006) or the time when the participants were institutionalized, dropped out, or died. Residence status, including survival, was obtained and verified through the official residential registry.
We note that the temporary absence was considered as community-dwelling. In addition, residential places, such as low-cost social welfare facilities and retirement homes, were not regarded as institutionalization because those residents were assumed to live relatively independently.
Aging-in-place preference
The explanatory variable was the aging-in-place preference. We converted the following eight desired places of care when bedridden into three places (i.e., facility, home, and other) and created two dummy variables with the facility as the reference category.
Participants were asked about their desired place of care when they were bedridden. Then, respondents chose one of the eight options: home with the hospital, nursing home, home with informal caregivers, home with formal caregivers, retirement housing, other, and do not know. In this study, we classified the responses into three preferences: (i) facility included two options (hospital and nursing home); (ii) home included three options (home with informal caregivers, home with formal caregivers, and retirement housing); and (iii) other included the remaining two options (other and do not know).
Covariates
Considering known predictors of institutionalization (3,4), we included several sociodemographic and health-related variables as covariates: age at the first survey, gender (0 = male, 1 = female), education (0 = less than 9 years, 1 = 10–12 years, and 2 = more than 13 years), perceived financial status (0 = extremely difficult to 4 = not at all difficult), living arrangement (0 = living alone, 1 = living with others), non-coresident children (0 = no, 1 = yes), and physical and cognitive function. To check the robustness of results on family networks, we also considered marital status (0 = not married, 1 = married) and co-resident children (0 = no, 1 = yes), instead of living arrangement.
Physical function was indexed as activities of daily living (ADLs). ADLs were assessed using ten activities (e.g., taking a bath, getting dressed, and moving in and out of bed), answered on a scale ranging from 0 = (cannot) to 4 = (not difficult). We calculated the summary score, ranging from 0 to 40. A higher value represents better physical function.
Cognitive function was assessed using the Short Portable Mental Status Questionnaire (SPMSQ) (19,20). Nine items were measured in the NSJE (e.g., memory, time and place orientation, and serial calculation). The number of correct answers was summed and used as the indicator of cognitive function. The score ranged from 0 to 9. A higher score indicates better cognitive function.
Statistical analysis
We first reported descriptive statistics and intercorrelations among the study variables. Next, we used Cox proportional hazards models and calculated hazard ratios (HRs) with 95% confidence intervals (CIs) to evaluate the association of aging-in-place preference when bedridden with institutionalization. The proportional hazard assumptions were graphically assessed using the Kaplan–Meier methods.
The percentage of respondents having missing information on the explanatory variable or covariates was 29.9%. To mitigate potential bias due to missing data, we applied the multiple imputation method. The method obtains unbiased estimates and standard errors when the missing at random assumption is satisfied (21). The imputation model included the explanatory variable and covariates at baseline. According to the guideline, we conducted 30 imputations, equivalent to the percentage of incomplete respondents. The underlying Markov chain was iterated ten times for each imputation. Then, we checked the imputation model by comparing the observed and imputed data (22). Finally, results were aggregated across the analyses to derive summary statistics by standard procedures (21). All statistical analyses were conducted using SPSS, version 28 (IBM Corp., Armonk, NY, USA).