Year	Years
1970	3.7
1971	3.8
1972	4
1973	4.1
1974	4.2
1975	4.4
1976	4.5
1977	4.6
1978	4.8
1979	4.9
1980	5.1
1981	5.2
1982	5.4
1983	5.5
1984	5.7
1985	5.8
1986	5.9
1987	6.1
1988	6.2
1989	6.4
1990	6.5
1991	6.7
1992	6.8
1993	6.9
1994	7.1
1995	7.2
1996	7.4
1997	7.5
1998	7.7
1999	7.8
2000	8
2001	8.1
2002	8.2
2003	8.4
2004	8.5
2005	8.7
2006	8.8
2007	8.9
2008	9.1
2009	9.2