Year	Years
1970	3.16
1971	3.25
1972	3.34
1973	3.43
1974	3.53
1975	3.62
1976	3.72
1977	3.82
1978	3.92
1979	4.03
1980	4.13
1981	4.24
1982	4.35
1983	4.46
1984	4.57
1985	4.68
1986	4.8
1987	4.91
1988	5.03
1989	5.15
1990	5.26
1991	5.38
1992	5.5
1993	5.62
1994	5.74
1995	5.87
1996	5.98
1997	6.1
1998	6.23
1999	6.36
2000	6.49
2001	6.62
2002	6.75
2003	6.89
2004	7.03
2005	7.17
2006	7.31
2007	7.46
2008	7.6
2009	7.75
2010	7.9
2011	8.05
2012	8.21
2013	8.36
2014	8.52
2015	8.67