Year	Years
1970	3.12
1971	3.22
1972	3.33
1973	3.44
1974	3.56
1975	3.67
1976	3.8
1977	3.92
1978	4.06
1979	4.19
1980	4.33
1981	4.48
1982	4.62
1983	4.78
1984	4.94
1985	5.1
1986	5.27
1987	5.44
1988	5.61
1989	5.79
1990	5.97
1991	6.16
1992	6.34
1993	6.53
1994	6.72
1995	6.9
1996	7.06
1997	7.22
1998	7.38
1999	7.55
2000	7.71
2001	7.88
2002	8.04
2003	8.21
2004	8.38
2005	8.55
2006	8.72
2007	8.89
2008	9.06
2009	9.23
2010	9.4
2011	9.57
2012	9.74
2013	9.91
2014	10.08
2015	10.25