Year	Years
1970	3.01
1971	3.1
1972	3.19
1973	3.28
1974	3.38
1975	3.48
1976	3.58
1977	3.68
1978	3.78
1979	3.89
1980	4
1981	4.11
1982	4.22
1983	4.34
1984	4.45
1985	4.57
1986	4.7
1987	4.82
1988	4.95
1989	5.08
1990	5.22
1991	5.35
1992	5.49
1993	5.63
1994	5.77
1995	5.92
1996	6.07
1997	6.22
1998	6.37
1999	6.52
2000	6.68
2001	6.83
2002	6.99
2003	7.15
2004	7.31
2005	7.47
2006	7.63
2007	7.79
2008	7.96
2009	8.12
2010	8.28
2011	8.45
2012	8.62
2013	8.78
2014	8.95
2015	9.12