Table 1. Basic demographic information for the three waves of the study population.
| Subgroup | 2011 | 2014 | 2018 |
| N =9,385 | N =6,719 | N =15,010 | |
| Functional impairment, n (%) | 2,500 (26.6) | 1,654 (24.6) | 3,980 (26.5) |
| Male, n (%) | 4,216 (44.9) | 3,080 (45.8) | 6,561 (43.7) |
| Rural, n (%) | 4,903 (52.2) | 3,619 (53.9) | 6,710 (44.7) |
| Age group, n (%) | |||
| 65–74 years | 1,882 (20.1) | 1,107 (16.5) | 3,116 (20.8) |
| 75–84 years | 2,476 (26.4) | 2,152 (32.0) | 4,069 (27.1) |
| 85+ years | 5,027 (53.6) | 3,460 (51.5) | 7,825 (52.1) |
| Attended school, n (%) | 3,911 (41.8) | 2,887 (43.3) | 6,415 (49.8) |
| Regions, n (%) | |||
| Eastern | 4,484 (47.8) | 3,223 (48.0) | 7,414 (49.4) |
| Central | 2,592 (27.6) | 1,960 (29.2) | 3,875 (25.8) |
| Western | 2,309 (24.6) | 1,536 (22.9) | 3,721 (24.8) |
| Household-income, n (%) | |||
| Low-income | 1,446 (20.0) | 1,184 (22.3) | 2,308 (20.5) |
| Low-middle-income | 1,641 (22.7) | 1,095 (20.6) | 2,336 (20.7) |
| Middle-income | 1,246 (17.3) | 1,235 (23.3) | 2,195 (19.5) |
| Middle-high-income | 1,439 (19.9) | 680 (12.8) | 2,385 (21.2) |
| High-income | 1,449 (20.1) | 1,114 (21.0) | 2,051 (18.2) |
| Married, n (%) | 3,540 (37.8) | 2,633 (39.6) | 6,075 (40.9) |
| Living alone, n (%) | 1,582 (17.0) | 1,247 (18.7) | 2,359 (15.9) |
| Having medical insurance, n (%) | 1,446 (20.0) | 1,184 (22.3) | 2,308 (20.5) |