Year	Years
1970	3.25
1971	3.32
1972	3.4
1973	3.48
1974	3.56
1975	3.64
1976	3.73
1977	3.81
1978	3.9
1979	3.99
1980	4.08
1981	4.18
1982	4.27
1983	4.37
1984	4.46
1985	4.56
1986	4.66
1987	4.77
1988	4.87
1989	4.98
1990	5.09
1991	5.2
1992	5.31
1993	5.43
1994	5.54
1995	5.66
1996	5.79
1997	5.91
1998	6.04
1999	6.16
2000	6.29
2001	6.42
2002	6.55
2003	6.68
2004	6.81
2005	6.95
2006	7.08
2007	7.22
2008	7.35
2009	7.49
2010	7.63
2011	7.77
2012	7.91
2013	8.05
2014	8.19
2015	8.33