Year	Years
1970	3.37
1971	3.47
1972	3.57
1973	3.67
1974	3.77
1975	3.87
1976	3.98
1977	4.09
1978	4.2
1979	4.31
1980	4.43
1981	4.55
1982	4.67
1983	4.79
1984	4.92
1985	5.04
1986	5.17
1987	5.3
1988	5.43
1989	5.57
1990	5.71
1991	5.85
1992	5.99
1993	6.13
1994	6.28
1995	6.42
1996	6.57
1997	6.72
1998	6.88
1999	7.03
2000	7.18
2001	7.34
2002	7.5
2003	7.66
2004	7.82
2005	7.98
2006	8.15
2007	8.31
2008	8.47
2009	8.64
2010	8.81
2011	8.97
2012	9.14
2013	9.31
2014	9.48
2015	9.65