Period | n | Mean | SD | SE |
1946-1958 | 13 | 20.2 | 1.3 | 0.4 |
1959-1982 | 24 | 19.4 | 0.9 | 0.2 |
1983-1990 | 8 | 18.9 | 0.4 | 0.2 |
1991-2008 | 18 | 19.8 | 0.6 | 0.1 |