Year	Years
1970	3.69
1971	3.79
1972	3.89
1973	3.99
1974	4.09
1975	4.2
1976	4.31
1977	4.43
1978	4.54
1979	4.66
1980	4.78
1981	4.91
1982	5.04
1983	5.17
1984	5.3
1985	5.44
1986	5.58
1987	5.72
1988	5.87
1989	6.01
1990	6.16
1991	6.32
1992	6.47
1993	6.63
1994	6.78
1995	6.94
1996	7.08
1997	7.22
1998	7.35
1999	7.49
2000	7.63
2001	7.76
2002	7.9
2003	8.04
2004	8.18
2005	8.31
2006	8.45
2007	8.59
2008	8.73
2009	8.87
2010	9
2011	9.14
2012	9.28
2013	9.42
2014	9.56
2015	9.7