Year	Years
1970	3.58
1971	3.67
1972	3.76
1973	3.85
1974	3.94
1975	4.03
1976	4.13
1977	4.23
1978	4.32
1979	4.43
1980	4.53
1981	4.64
1982	4.74
1983	4.85
1984	4.96
1985	5.08
1986	5.19
1987	5.31
1988	5.43
1989	5.55
1990	5.68
1991	5.81
1992	5.93
1993	6.06
1994	6.2
1995	6.33
1996	6.47
1997	6.6
1998	6.74
1999	6.88
2000	7.02
2001	7.16
2002	7.3
2003	7.44
2004	7.58
2005	7.72
2006	7.87
2007	8.01
2008	8.15
2009	8.3
2010	8.44
2011	8.58
2012	8.73
2013	8.87
2014	9.01
2015	9.16