Year	Years
1970	2.45
1971	2.52
1972	2.58
1973	2.65
1974	2.71
1975	2.78
1976	2.85
1977	2.93
1978	3
1979	3.08
1980	3.15
1981	3.23
1982	3.31
1983	3.39
1984	3.48
1985	3.56
1986	3.65
1987	3.74
1988	3.83
1989	3.92
1990	4.02
1991	4.12
1992	4.21
1993	4.32
1994	4.42
1995	4.52
1996	4.64
1997	4.75
1998	4.86
1999	4.97
2000	5.09
2001	5.21
2002	5.32
2003	5.44
2004	5.56
2005	5.68
2006	5.8
2007	5.93
2008	6.05
2009	6.17
2010	6.3
2011	6.42
2012	6.55
2013	6.68
2014	6.81
2015	6.93