Overview of Survey Design
Survey participants and methods
A total of nine surveys were conducted between 1987 and 2017.
All participants newly added to the survey were randomly selected throughout Japan using a technique called two-stage stratified random sampling, although the year they were first added varied depending on the participant (as shown in Fig. 1). Note
(Note) Regions were stratified according to a combination of regional block (Hokkaido, Tohoku, Kanto, etc.) and population, and the number of persons (sample size) selected from each stratum was determined depending on the population of the target ages in the stratum. The procedure for extracting samples involved first selecting 192 survey locations throughout the country, then selecting individuals of the relevant ages from each location using the Basic Resident Register.
Participants who joined the survey for the first time were generally asked to participate in each subsequent survey, unless they died or declined to continue participating. Whether they actually participate is of course left to the discretion of the individual. By the ninth survey, 6,665 individuals had participated at least once.
Central Research Services, Inc. conducted visit interviews from September to December of each year of the survey. The surveys were generally conducted as interviews of the participants themselves. Beginning from the second survey, however, other individuals (proxies) such as family members were surveyed (for certain items) only if the participant was unable to respond for a reason such as serious illness.
The following describes the participants of each survey in somewhat greater detail.
In the first wave (1987), responses were obtained from 2,200 individuals aged 60 years or older throughout Japan (response rate, 67.2%; A of Fig. 1). Follow-up surveys were subsequently conducted every 3 years. Individuals aged 60-62 years and individuals aged 60-65 years were newly selected at random and supplemented to the panel in the second (1990, B of Fig. 1) and fourth (1996, C of Fig. 1) waves, respectively.
In the second and fourth waves, the number of newly selected individuals (sample size) was specified (second wave: 580, fourth wave: 1,210) so that the selection probability for an age group was roughly equal to its previous selection probability for an older age group. This was done so that the data analyzed were an unbiased reflection of individuals aged 60 years or older throughout Japan.
By the fourth wave, 3,580 individuals had participated (including proxies) and were asked to be participants in subsequent follow-up surveys.
In the fifth wave conducted in 1999, 2,000 new individuals aged 70 years or older were randomly selected (D of Fig. 1), while continuing follow-up of the A to C participants who had cooperated through the fourth wave. Individuals aged 70 years or older were added because the issues of the older old were a focus of the survey (→ see About the Project). Of the 2,000 new individuals, 1,635 who responded were added to the follow-up panel.
The sixth (2002) and seventh (2006) waves continued the follow-up of the A to D participants, without supplementing new samples.
In the eighth wave (2012), 2,500 individuals aged 60-92 years (E of Fig. 1) were randomly selected, while continuing the follow-up of the A to D participants. Of the new E participants who were respondents in the eighth wave (n=1,450), proxy participants and those who did not consent to subsequent participation were excluded from follow-up, and 1,257 remained as follow-up panel. The ninth wave (2017) continued follow-up of the A to E participants, without supplementing new samples.
Number of respondents and response rate
The number of respondents (including proxy respondents) for each wave ranged from 2,000 to 4,000 through the eighth wave, but dropped below 2,000 for the first time in the ninth wave. The decrease occurred because death increased as the participants aged. The number of participants confirmed to have died up to the ninth wave (2017) was 2,426 for the A to C participants in Fig. 1 (of 3,580 follow-up participants), 1,133 for the D participants (of 1,635 follow-up participants), and 110 for the E participants (of 1,257 follow-up participants).
The response rate was 67% in the first wave (not including proxy response for the first wave), approximately 85% in the second to sixth waves, and approximately 75% in the seventh and ninth waves. However, it should be noted that comparing the response rates across waves is difficult because the presence or absence and proportion of newly added participants and the ages of the participants varied. In the eighth wave, the response rate for the newly added E participants (Fig. 1) was 59%, the lowest seen so far, and the overall response rate for the A to E participants was 67%.
For more information, please see Table 2.
The questions asked in the interview surveys of the participants are broad in scope. They comprise questions about physical and mental health, family, relationships with friends and neighbors, work and social participation, lifestyle, use of medical/health and welfare services, and economic conditions. Although many of the questions concern the status of the participant's health and life, there are also questions about their views, such as how they feel about their family.
Moreover, most of the questions are asked repeatedly in every survey or periodically (once every few surveys). On the other hand, there have also been questions that were asked only in certain surveys in order to respond to the social conditions and research trends present at that time.
With regard to physical condition, in addition to reporting by the participants themselves (responses to questions), the interviewers took objective measures of the body and physical performance, including height, weight, walking speed, and grip strength. The measurements were first taken in the eighth wave survey and continued in the ninth.
◎ Details on the methods used in the eighth wave are provided in the following study report (in Japanese):
Longitudinal study of the health and lives of older adults: Wave 8 survey (2012) research report (March 2015), Tokyo Metropolitan Institute of Gerontology
- Section 1: Project summary and the participants and methods of the eighth survey (pp. 1-20)
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