1950 31.2
 1951 37.
 1952 25.1
 1953 33.9
 1954 44.2
 1955 30.4
 1956 42.2
 1957 63.5
 1958 38.2
 1959 45.5
 1960 29.300001
 1961 32.600002
 1962 27.
 1963 59.8
 1964 22.6
 1965 40.4
 1966 34.4
 1967 20.
 1968 24.5
 1969 28.9
 1970 25.300001
 1971 41.5
 1972 87.
 1973 30.6
 1974 23.2
 1975 29.7
 1976 25.300001
 1977 33.
 1978 34.
 1979 36.2
 1980 30.
 1981 24.7
 1982 46.
 1983 26.7
 1984 31.800001
 1985 22.5
 1986 37.100002
 1987 35.2
 1988 31.2
 1989 34.2
 1990 23.9
 1991 26.9
 1992 18.6
 1993 36.600002
 1994 20.800001
 1995 29.
 1996 25.9
 1997 50.2
 1998 29.5
 1999 22.6
 2000 23.5
 2001 23.5
 2002 24.
 2003 34.3
 2004 35.3
 2005 23.
 2006 35.
 2007 50.