Year	Years
1970	2.6
1971	2.8
1972	2.9
1973	3
1974	3.2
1975	3.3
1976	3.5
1977	3.6
1978	3.8
1979	4
1980	4.1
1981	4.3
1982	4.5
1983	4.7
1984	4.9
1985	5
1986	5.2
1987	5.4
1988	5.6
1989	5.7
1990	5.9
1991	6.1
1992	6.2
1993	6.4
1994	6.5
1995	6.7
1996	6.9
1997	7
1998	7.2
1999	7.4
2000	7.5
2001	7.7
2002	7.8
2003	8
2004	8.2
2005	8.3
2006	8.5
2007	8.6
2008	8.8
2009	8.9