Year	Years
1970	2.18
1971	2.24
1972	2.3
1973	2.36
1974	2.42
1975	2.48
1976	2.55
1977	2.61
1978	2.68
1979	2.75
1980	2.82
1981	2.89
1982	2.97
1983	3.04
1984	3.12
1985	3.2
1986	3.28
1987	3.36
1988	3.45
1989	3.54
1990	3.63
1991	3.72
1992	3.81
1993	3.91
1994	4
1995	4.11
1996	4.22
1997	4.33
1998	4.45
1999	4.56
2000	4.68
2001	4.79
2002	4.91
2003	5.03
2004	5.15
2005	5.27
2006	5.39
2007	5.51
2008	5.63
2009	5.76
2010	5.88
2011	6.01
2012	6.13
2013	6.26
2014	6.38
2015	6.51