Year	Years
1970	3.69
1971	3.78
1972	3.88
1973	3.97
1974	4.07
1975	4.18
1976	4.28
1977	4.39
1978	4.5
1979	4.61
1980	4.72
1981	4.84
1982	4.95
1983	5.07
1984	5.19
1985	5.32
1986	5.44
1987	5.57
1988	5.7
1989	5.83
1990	5.96
1991	6.09
1992	6.22
1993	6.36
1994	6.49
1995	6.63
1996	6.74
1997	6.86
1998	6.98
1999	7.1
2000	7.22
2001	7.34
2002	7.46
2003	7.59
2004	7.71
2005	7.84
2006	7.97
2007	8.09
2008	8.23
2009	8.36
2010	8.49
2011	8.62
2012	8.76
2013	8.89
2014	9.03
2015	9.17