Comparison of data from five sources for consecutive patients with RA
| Institution | Period | City | Area | Patients (n) | Peak age at onset | Mean disease duration (y) |
| Group I; Osaka University Hospital (Dept Orthop Surg) | 1960–1965 | Osaka | urban | 566 | 20s | 4.9 |
| Group II; Shichikawa Arthritis Research Centre (Dept Rheumatol) | 1985–1990 | Hisai | rural | 366 | 40s | 7.5 |
| Osaka University Hospital (Dept Orthop Surg) | 1985–1990 | Osaka | urban | 286 | 40s | 5.7 |
| Yukioka Hospital (Dept Rheumatol) | 1985–1990 | Osaka | urban | 233 | 40s | — |
| Osaka-Minami National Hospital (Dept Orthop Surg) | 1991–1994 | Kawachi-Nagano | suburban | 370 | 40s | 11.6 |