Year	Years
1970	1.6
1971	1.7
1972	1.8
1973	2
1974	2.1
1975	2.2
1976	2.3
1977	2.5
1978	2.6
1979	2.8
1980	2.9
1981	3.1
1982	3.2
1983	3.4
1984	3.6
1985	3.7
1986	3.9
1987	4.1
1988	4.3
1989	4.4
1990	4.6
1991	4.8
1992	5
1993	5.2
1994	5.4
1995	5.6
1996	5.8
1997	6
1998	6.2
1999	6.4
2000	6.6
2001	6.8
2002	7
2003	7.2
2004	7.4
2005	7.6
2006	7.8
2007	8
2008	8.2
2009	8.5