1950 36.5
 1951 38.
 1952 36.600002
 1953 34.2
 1954 35.
 1955 33.600002
 1956 34.600002
 1957 38.
 1958 37.5
 1959 37.2
 1960 36.
 1961 34.600002
 1962 35.100002
 1963 33.600002
 1964 34.5
 1965 37.3
 1966 38.4
 1967 36.8
 1968 36.2
 1969 35.
 1970 35.
 1971 35.2
 1972 33.2
 1973 35.8
 1974 36.3
 1975 36.4
 1976 36.600002
 1977 34.2
 1978 37.600002
 1979 36.5
 1980 37.
 1981 38.
 1982 38.
 1983 36.4
 1984 37.4
 1985 37.600002
 1986 37.5
 1987 39.
 1988 37.
 1989 37.2
 1990 37.
 1991 39.4
 1992 37.9
 1993 40.
 1994 38.
 1995 39.5
 1996 37.
 1997 36.5
 1998 36.5
 1999 37.3
 2000 36.4
 2001 36.4
 2002 36.3
 2003 38.600002
 2004 38.4
 2005 39.600002
 2006 38.5
 2007 39.