Year	Years
1970	3.82
1971	3.91
1972	4
1973	4.08
1974	4.17
1975	4.26
1976	4.36
1977	4.45
1978	4.54
1979	4.64
1980	4.74
1981	4.84
1982	4.94
1983	5.04
1984	5.14
1985	5.25
1986	5.35
1987	5.46
1988	5.57
1989	5.68
1990	5.79
1991	5.9
1992	6.02
1993	6.13
1994	6.25
1995	6.37
1996	6.49
1997	6.61
1998	6.74
1999	6.86
2000	6.99
2001	7.12
2002	7.25
2003	7.38
2004	7.51
2005	7.64
2006	7.77
2007	7.91
2008	8.04
2009	8.17
2010	8.31
2011	8.45
2012	8.58
2013	8.72
2014	8.86
2015	9