1950 3.8
1955 4.9
1960 6.1
1961 6.4
1962 6.7
1963 7.0
1964 7.5
1965 7.9
1966 8.3
1967 8.6
1968 9.1
1969 9.7
1970 10.1
1971 10.5
1972 11.0
1973 11.7
1974 11.8
1975 11.9
1976 12.5
1977 13.0
1978 13.5
1979 14.0 
1980 14.1
1981 14.3
1982 14.4
1983 14.8
1984 15.4
1985 16.0
1986 16.4
1987 17.0
1988 17.8
1989 18.4
1990 18.8
1991 18.7
1992 18.9
1993 19.3