|---|---|
| 1915 | 0.27 |
| 1920 | 0.31 |
| 1925 | 0.21 |
| 1930 | 0.18 |
| 1935 | 0.13 |
| 1940 | 0.18 |
| 1945 | 0.21 |
| 1950 | 0.27 |
| 1955 | 0.30 |
| 1960 | 0.31 |
| 1965 | 0.32 |
| 1970 | 0.36 |
| 1975 | 0.56 |
| 1980 | 1.21 |
| 1985 | 1.10 |
| 1990 | 1.17 |
| 1995 | 1.15 |
| 2000 | 1.51 |
| 2005 | 2.33 |
| 2010 | 2.80 |
