Year	Years
1970	3.1
1971	3.3
1972	3.4
1973	3.5
1974	3.7
1975	3.8
1976	3.9
1977	4.1
1978	4.2
1979	4.4
1980	4.5
1981	4.7
1982	4.8
1983	5
1984	5.2
1985	5.3
1986	5.5
1987	5.6
1988	5.8
1989	6
1990	6.1
1991	6.3
1992	6.4
1993	6.6
1994	6.8
1995	6.9
1996	7.1
1997	7.2
1998	7.4
1999	7.6
2000	7.7
2001	7.9
2002	8
2003	8.2
2004	8.3
2005	8.5
2006	8.6
2007	8.8
2008	8.9
2009	9