Year	Years
1970	3.54
1971	3.62
1972	3.71
1973	3.79
1974	3.88
1975	3.97
1976	4.06
1977	4.16
1978	4.25
1979	4.35
1980	4.45
1981	4.55
1982	4.65
1983	4.75
1984	4.86
1985	4.96
1986	5.07
1987	5.18
1988	5.29
1989	5.41
1990	5.52
1991	5.64
1992	5.76
1993	5.88
1994	6
1995	6.12
1996	6.24
1997	6.37
1998	6.5
1999	6.63
2000	6.75
2001	6.88
2002	7.01
2003	7.14
2004	7.28
2005	7.41
2006	7.54
2007	7.68
2008	7.81
2009	7.94
2010	8.08
2011	8.22
2012	8.35
2013	8.49
2014	8.63
2015	8.77