Year	Years
1970	3.45
1971	3.53
1972	3.62
1973	3.71
1974	3.8
1975	3.9
1976	3.99
1977	4.08
1978	4.18
1979	4.27
1980	4.37
1981	4.46
1982	4.56
1983	4.65
1984	4.75
1985	4.85
1986	4.94
1987	5.04
1988	5.13
1989	5.22
1990	5.32
1991	5.41
1992	5.5
1993	5.59
1994	5.68
1995	5.76
1996	5.88
1997	6
1998	6.13
1999	6.25
2000	6.38
2001	6.52
2002	6.65
2003	6.79
2004	6.93
2005	7.07
2006	7.21
2007	7.36
2008	7.51
2009	7.66
2010	7.81
2011	7.97
2012	8.12
2013	8.28
2014	8.44
2015	8.6