Year	Years
1970	3.71
1971	3.81
1972	3.91
1973	4.01
1974	4.11
1975	4.22
1976	4.33
1977	4.44
1978	4.55
1979	4.67
1980	4.79
1981	4.91
1982	5.03
1983	5.15
1984	5.28
1985	5.4
1986	5.53
1987	5.66
1988	5.79
1989	5.92
1990	6.05
1991	6.19
1992	6.32
1993	6.45
1994	6.58
1995	6.72
1996	6.83
1997	6.95
1998	7.07
1999	7.19
2000	7.31
2001	7.42
2002	7.54
2003	7.66
2004	7.78
2005	7.9
2006	8.02
2007	8.14
2008	8.26
2009	8.38
2010	8.51
2011	8.63
2012	8.75
2013	8.88
2014	9
2015	9.13