Year	Years
1970	4.6
1971	4.7
1972	4.9
1973	5
1974	5.2
1975	5.3
1976	5.5
1977	5.6
1978	5.8
1979	5.9
1980	6.1
1981	6.3
1982	6.4
1983	6.6
1984	6.7
1985	6.9
1986	7
1987	7.2
1988	7.4
1989	7.5
1990	7.7
1991	7.8
1992	8
1993	8.1
1994	8.3
1995	8.4
1996	8.6
1997	8.7
1998	8.8
1999	9
2000	9.1
2001	9.3
2002	9.4
2003	9.5
2004	9.7
2005	9.8
2006	9.9
2007	10.1
2008	10.2
2009	10.3