Year	Years
1970	3.49
1971	3.58
1972	3.67
1973	3.77
1974	3.87
1975	3.96
1976	4.06
1977	4.16
1978	4.27
1979	4.37
1980	4.47
1981	4.58
1982	4.68
1983	4.79
1984	4.9
1985	5
1986	5.11
1987	5.22
1988	5.33
1989	5.44
1990	5.55
1991	5.66
1992	5.76
1993	5.87
1994	5.98
1995	6.09
1996	6.22
1997	6.35
1998	6.49
1999	6.63
2000	6.77
2001	6.91
2002	7.06
2003	7.21
2004	7.36
2005	7.51
2006	7.66
2007	7.82
2008	7.97
2009	8.13
2010	8.29
2011	8.45
2012	8.62
2013	8.78
2014	8.94
2015	9.11