Year	Years
1970	3.56
1971	3.64
1972	3.72
1973	3.81
1974	3.89
1975	3.98
1976	4.07
1977	4.16
1978	4.25
1979	4.34
1980	4.43
1981	4.53
1982	4.62
1983	4.72
1984	4.82
1985	4.91
1986	5.01
1987	5.11
1988	5.22
1989	5.32
1990	5.42
1991	5.53
1992	5.63
1993	5.74
1994	5.85
1995	5.96
1996	6.07
1997	6.19
1998	6.31
1999	6.43
2000	6.55
2001	6.67
2002	6.8
2003	6.92
2004	7.05
2005	7.17
2006	7.3
2007	7.43
2008	7.56
2009	7.69
2010	7.82
2011	7.95
2012	8.09
2013	8.22
2014	8.35
2015	8.49