Year	Years
1970	2.3
1971	2.5
1972	2.6
1973	2.7
1974	2.8
1975	2.9
1976	3.1
1977	3.2
1978	3.3
1979	3.5
1980	3.6
1981	3.7
1982	3.9
1983	4
1984	4.2
1985	4.3
1986	4.5
1987	4.6
1988	4.8
1989	4.9
1990	5.1
1991	5.2
1992	5.4
1993	5.5
1994	5.7
1995	5.9
1996	6
1997	6.2
1998	6.4
1999	6.6
2000	6.7
2001	6.9
2002	7.1
2003	7.3
2004	7.5
2005	7.7
2006	7.9
2007	8.1
2008	8.2
2009	8.4