Year	Years
1970	3.55
1971	3.63
1972	3.72
1973	3.81
1974	3.9
1975	3.99
1976	4.08
1977	4.17
1978	4.27
1979	4.37
1980	4.47
1981	4.57
1982	4.67
1983	4.77
1984	4.88
1985	4.99
1986	5.09
1987	5.21
1988	5.32
1989	5.43
1990	5.55
1991	5.66
1992	5.78
1993	5.9
1994	6.02
1995	6.15
1996	6.27
1997	6.39
1998	6.51
1999	6.64
2000	6.76
2001	6.89
2002	7.02
2003	7.15
2004	7.28
2005	7.41
2006	7.54
2007	7.67
2008	7.8
2009	7.94
2010	8.07
2011	8.21
2012	8.34
2013	8.48
2014	8.62
2015	8.75