1950 46 18.0
1955 60 21.6
1960 68 22.4
1961 70 22.7
1962 73 23.3
1963 77 24.0
1964 78 23.8
1965 82 24.5
1966 86 25.2
1967 90 25.8
1968 93 26.2
1969 94 25.9
1970 98 26.5
1971 102 27.0
1972 105 27.2
1973 106 26.9
1974 112 27.9
1975 113 27.7
1976 116 27.9
1977 120 28.4
1978 125 29.0
1979 129 29.5
1980  133 29.9
1981 136 30.0
1982 137 29.7
1983 142 30.3
1984 145 30.4
1985 150 30.9
1986 156 31.6
1987 161 32.0
1988 164 32.1
1989 167 32.1
1990  171 32.3
1991 174 32.3
1992 176 32.1
1993 179 32.2