1950 22 8.6
1955 29 10.4
1960 38 12.5
1961 42 13.6
1962 45 14.4
1963 48 15.0
1964 53 16.2
1965 54 16.1
1966 57 16.7
1967 60 17.2
1968 64 18.0
1969 63 17.4
1970  66 17.8
1971 66 17.5
1972 62 16.1
1973 63 16.0
1974 67 16.7
1975 66 16.2
1976 69 16.6
1977 70 16.3
1978 70 16.3
1979 71 16.2
1980 72 16.2
1981 75 16.6
1982 77 16.7
1983 77 16.4
1984 84 17.6
1985 86 17.7
1986 93 18.8
1987 94 18.7
1988 99 19.4
1989 100 19.2
1990  97 18.3
1991 97 18.0
1992 98 17.9
1993 98 17.6