Year	Years
1970	3.28
1971	3.36
1972	3.44
1973	3.52
1974	3.61
1975	3.69
1976	3.78
1977	3.87
1978	3.96
1979	4.05
1980	4.15
1981	4.24
1982	4.34
1983	4.44
1984	4.54
1985	4.65
1986	4.75
1987	4.86
1988	4.97
1989	5.08
1990	5.2
1991	5.31
1992	5.43
1993	5.55
1994	5.67
1995	5.8
1996	5.94
1997	6.08
1998	6.22
1999	6.36
2000	6.5
2001	6.64
2002	6.78
2003	6.92
2004	7.06
2005	7.2
2006	7.34
2007	7.49
2008	7.63
2009	7.77
2010	7.92
2011	8.06
2012	8.2
2013	8.35
2014	8.49
2015	8.63