Year	Years
1970	3.92
1971	4.01
1972	4.11
1973	4.21
1974	4.31
1975	4.41
1976	4.51
1977	4.62
1978	4.72
1979	4.83
1980	4.94
1981	5.05
1982	5.16
1983	5.27
1984	5.39
1985	5.5
1986	5.62
1987	5.74
1988	5.85
1989	5.97
1990	6.09
1991	6.21
1992	6.33
1993	6.45
1994	6.56
1995	6.68
1996	6.8
1997	6.91
1998	7.03
1999	7.15
2000	7.26
2001	7.38
2002	7.5
2003	7.62
2004	7.74
2005	7.86
2006	7.98
2007	8.1
2008	8.23
2009	8.35
2010	8.47
2011	8.6
2012	8.73
2013	8.85
2014	8.98
2015	9.11