Year	Years
1970	3.75
1971	3.86
1972	3.97
1973	4.08
1974	4.2
1975	4.32
1976	4.44
1977	4.56
1978	4.68
1979	4.81
1980	4.94
1981	5.07
1982	5.21
1983	5.35
1984	5.49
1985	5.63
1986	5.78
1987	5.93
1988	6.08
1989	6.23
1990	6.39
1991	6.56
1992	6.73
1993	6.9
1994	7.07
1995	7.26
1996	7.41
1997	7.57
1998	7.73
1999	7.9
2000	8.06
2001	8.22
2002	8.39
2003	8.55
2004	8.72
2005	8.88
2006	9.05
2007	9.22
2008	9.39
2009	9.55
2010	9.72
2011	9.89
2012	10.06
2013	10.23
2014	10.4
2015	10.56