1948 16.6
1949 16.2
1950 15.2
1951 14.2
1952 13.9
1953 12.5
1954 11.7
1955 11.5
1956 11.1
1957 10.3
1958 9.8
1959 9.3
1960 8.7
1961 8.1
1962 7.7
1963 7.1
1964 6.7
1965 6.4
1966 5.9
1967 5.5
1968 5.2
1969 5.1
1970 4.7
1971 4.5
1972 4.6
1973 4.5
1974 4.3
1975 4.1
1976 3.8
1977 2.8
1978 2.9
1979 2.8
1980 2.7
1981 2.5
1982 2.4
1983 2.5
1984 2.4
1985 2.2
1986 2.2
1987 2.1
1988 2.1
1989 2.0
1990 1.9
1991 1.9