Year	Years
1970	2.46
1971	2.52
1972	2.59
1973	2.66
1974	2.72
1975	2.79
1976	2.86
1977	2.94
1978	3.01
1979	3.08
1980	3.16
1981	3.23
1982	3.31
1983	3.39
1984	3.47
1985	3.56
1986	3.64
1987	3.72
1988	3.81
1989	3.89
1990	3.98
1991	4.07
1992	4.15
1993	4.24
1994	4.33
1995	4.41
1996	4.49
1997	4.58
1998	4.67
1999	4.76
2000	4.85
2001	4.94
2002	5.04
2003	5.14
2004	5.24
2005	5.34
2006	5.44
2007	5.55
2008	5.66
2009	5.77
2010	5.88
2011	5.99
2012	6.11
2013	6.22
2014	6.34
2015	6.46