Year	Years
1970	3.68
1971	3.77
1972	3.85
1973	3.94
1974	4.03
1975	4.12
1976	4.21
1977	4.31
1978	4.4
1979	4.5
1980	4.59
1981	4.69
1982	4.79
1983	4.89
1984	4.99
1985	5.09
1986	5.19
1987	5.29
1988	5.39
1989	5.49
1990	5.6
1991	5.7
1992	5.8
1993	5.91
1994	6.01
1995	6.11
1996	6.22
1997	6.32
1998	6.43
1999	6.54
2000	6.65
2001	6.76
2002	6.87
2003	6.99
2004	7.1
2005	7.22
2006	7.34
2007	7.46
2008	7.58
2009	7.71
2010	7.83
2011	7.96
2012	8.08
2013	8.21
2014	8.34
2015	8.47