Year	Years
1970	3.43
1971	3.53
1972	3.62
1973	3.72
1974	3.82
1975	3.93
1976	4.04
1977	4.15
1978	4.26
1979	4.38
1980	4.5
1981	4.63
1982	4.76
1983	4.89
1984	5.02
1985	5.16
1986	5.3
1987	5.44
1988	5.59
1989	5.74
1990	5.89
1991	6.05
1992	6.2
1993	6.36
1994	6.51
1995	6.66
1996	6.79
1997	6.92
1998	7.05
1999	7.18
2000	7.31
2001	7.44
2002	7.57
2003	7.7
2004	7.83
2005	7.97
2006	8.1
2007	8.24
2008	8.37
2009	8.51
2010	8.65
2011	8.78
2012	8.92
2013	9.06
2014	9.2
2015	9.34