| YEAR | % | RATE |
| **1938 | 66.7 | 20.75 |
| 1939 | 66.7 | 21.50 |
| 1940 | 66.7 | 23.25 |
| 1941 | 66.7 | 20.25 |
| 1942 | 66.7 | 20.25 |
| 1943 | 66.7 | 21.00 |
| 1944 | 66.7 | 22.00 |
| 1945 | 66.7 | 21.00 |
| 1946 | 66.7 | 21.00 |
| 1947 | 66.7 | 22.00 |
| 1948 | 66.7 | 21.00 |
| 1949 | 66.7 | 21.00 |
| 1950 | 66.7 | 26.00 |
| 1951 | 66.7 | 27.00 |
| 1952 | 66.7 | 34.00 |
| **1953 | 50.0 | 27.00 |
| 1954 | 50.0 | 26.00 |
| 1955 | 50.0 | 28.50 |
| 1956 | 50.0 | 34.00 |
| 1957 | 50.0 | 29.50 |
| 1958 | 50.0 | 29.50 |
| 1959 | 50.0 | 33.50 |
| *1960 | 50.0 | 33.50 |
| 1961 | 66.7 | 40.00 |
| 1962 | 66.7 | 43.00 |
| 1963 | 66.7 | 51.00 |
| 1964 | 66.7 | 47.00 |
| 1965 | 66.7 | 47.00 |
| 1966 | 66.7 | 47.00 |
| 1967 | 66.7 | 47.00 |
| 1968 | 66.7 | 47.00 |
| **1969 | 65.0 | 33.00 |
| 1970 | 65.0 | 33.00 |
| 1971 | 65.0 | 35.00 |
| 1972 | 65.0 | 35.00 |
| 1973 | 65.0 | 35.00 |
| 1974 | 65.0 | 37.00 |
| 1975 | 65.0 | 37.00 |
| 1976 | 65.0 | 38.00 |
| 1977 | 65.0 | 38.00 |
| 1978 | 65.0 | 38.00 |
| 1979 | 65.0 | 40.00 |
| **1980 | 70.0 | 19.00 |
| 1981 | 70.0 | 21.00 |
| 1982 | 70.0 | 22.50 |
| 1983 | 70.0 | 23.70 |
| 1984 | 70.0 | 23.30 |
| 1985 | 70.0 | 26.00 |
| 1986 | 70.0 | 26.50 |
| 1987 | 70.0 | 28.50 |
| 1988 | 70.0 | 30.25 |
| 1989 | 70.0 | 32.75 |
| 1990 | 70.0 | 36.75 |
| **1991 | 70.0 | 20.60 |
| 1992 | 70.0 | 20.30 |
| 1993 | 70.0 | 21.40 |
| 1994 | 70.0 | 21.80 |
| 1995 | 70.0 | 22.19 |
| 1996 | 70.0 | 22.56 |
| 1997 | 70.0 | 23.53 |
| 1998 | 70.0 | 24.89 |
| 1999 | 70.0 | 26.19 |
| 2000 | 70.0 | 27.92 |
| 2001 | 70.0 | 29.14 |
| **2002 | 70.0 | 22.79 |
| 2003 | 70.0 | 23.94 |
| **2004 | 70.0 | 20.97 |
| 2005 | 70.0 | 21.76 |
| 2006 | 70.0 | 22.33 |
| 2007 | 70.0 | 23.21 |
| 2008 | 70.0 | 23.58 |
| **2009 | 70.0 | 23.57 |
| 2010 | 70.0 | 24.27 |
| 2011 | 70.0 | 24.95 |
| 2012 | 70.0 | 25.59 |
| 2013 | 70.0 | 26.24 |
| **2014 | 70.0 | 26.93 |
| 2015 | 70.0 | 27.41 |
| 2016 | 70.0 | 28.47 |
| 2017 | 70.0 | 28.64 |