Year	Years
1970	2.4
1971	2.5
1972	2.6
1973	2.8
1974	2.9
1975	3.1
1976	3.2
1977	3.4
1978	3.6
1979	3.7
1980	3.9
1981	4.1
1982	4.2
1983	4.4
1984	4.6
1985	4.8
1986	5
1987	5.1
1988	5.3
1989	5.5
1990	5.7
1991	5.9
1992	6.1
1993	6.2
1994	6.4
1995	6.6
1996	6.8
1997	7
1998	7.2
1999	7.4
2000	7.5
2001	7.7
2002	7.9
2003	8.1
2004	8.3
2005	8.4
2006	8.6
2007	8.8
2008	8.9
2009	9.1