Year	Years
1970	2.6
1971	2.7
1972	2.8
1973	2.9
1974	3.1
1975	3.2
1976	3.3
1977	3.5
1978	3.6
1979	3.8
1980	3.9
1981	4.1
1982	4.2
1983	4.4
1984	4.6
1985	4.7
1986	4.9
1987	5.1
1988	5.3
1989	5.4
1990	5.6
1991	5.8
1992	6
1993	6.2
1994	6.3
1995	6.5
1996	6.7
1997	6.9
1998	7.1
1999	7.3
2000	7.4
2001	7.6
2002	7.8
2003	8
2004	8.2
2005	8.4
2006	8.5
2007	8.7
2008	8.9
2009	9.1