1960 1.4
1961 1.3
1962 1.3
1963 1.3
1970 2.5
1974 2.4
1976 2.8
1977 3.3
1978 4.6
1979 5.7
1980 8.2
1981 9.8
1982 10.4
1983 10.9
1984 10.5
1985 11.6
1986 12.4
1987 13.3
1988 14.8
1989 14.9
1990 17.2
1991 17.0
1992 18.2
1993 19.0