Year	Years
1970	2.35
1971	2.42
1972	2.5
1973	2.57
1974	2.65
1975	2.74
1976	2.82
1977	2.91
1978	3
1979	3.09
1980	3.18
1981	3.28
1982	3.38
1983	3.48
1984	3.59
1985	3.69
1986	3.81
1987	3.92
1988	4.04
1989	4.16
1990	4.28
1991	4.4
1992	4.53
1993	4.66
1994	4.8
1995	4.94
1996	5.07
1997	5.21
1998	5.35
1999	5.49
2000	5.63
2001	5.77
2002	5.92
2003	6.07
2004	6.22
2005	6.37
2006	6.52
2007	6.68
2008	6.83
2009	6.99
2010	7.15
2011	7.31
2012	7.47
2013	7.63
2014	7.79
2015	7.95