| Other higher | No difference/similar | Other lower | Variable# | Totals | |
| 1920s | 0 (0.0) | 1 (14.3) | 6 (85.7) | 0 (0.0) | 7 (100.0) |
| 1930s | 1 (14.3) | 0 (0.0) | 6 (85.7) | 0 (0.0) | 7 (100.0) |
| 1940s | 0 (0.0) | 0 (0.0) | 2 (100.0) | 0 (0.0) | 2 (100.0) |
| 1950s | 0 (0.0) | 0 (0.0) | 2 (100.0) | 0 (0.0) | 2 (100.0) |
| 1960s | 1 (5.3) | 0 (0.0) | 16 (84.2) | 2 (10.5) | 19 (100.0) |
| 1970s | 2 (4.0) | 1 (2.0) | 46 (92.0) | 1 (2.0) | 50 (100.0) |
| 1980s | 3 (5.7) | 3 (5.7) | 42 (79.2) | 5 (9.4) | 53 (100.0) |
| 1990s | 0 (0.0) | 5 (9.6) | 43 (82.7) | 4 (7.7) | 52 (100.0) |
| 2000s | 3 (8.8) | 3 (8.8) | 26 (76.5) | 2 (5.9) | 34 (100.0) |
| All years | 10 (4.4) | 13 (5.8) | 189 (83.6) | 14 (6.2) | 226 (100.0) |
Data are presented as n (%). #: the difference detected was found to be variable by age, sex, race/ethnicity, equation, spirometry method or percentage.