Year	Years
1970	3.53
1971	3.62
1972	3.72
1973	3.81
1974	3.91
1975	4.01
1976	4.12
1977	4.22
1978	4.33
1979	4.44
1980	4.56
1981	4.67
1982	4.79
1983	4.92
1984	5.04
1985	5.17
1986	5.3
1987	5.44
1988	5.57
1989	5.71
1990	5.86
1991	6
1992	6.15
1993	6.29
1994	6.44
1995	6.59
1996	6.72
1997	6.85
1998	6.99
1999	7.12
2000	7.26
2001	7.39
2002	7.53
2003	7.66
2004	7.8
2005	7.94
2006	8.08
2007	8.22
2008	8.36
2009	8.5
2010	8.64
2011	8.78
2012	8.92
2013	9.06
2014	9.2
2015	9.34