Year	Years
1970	2.46
1971	2.52
1972	2.58
1973	2.65
1974	2.71
1975	2.78
1976	2.85
1977	2.91
1978	2.99
1979	3.06
1980	3.13
1981	3.21
1982	3.28
1983	3.36
1984	3.44
1985	3.52
1986	3.6
1987	3.69
1988	3.77
1989	3.86
1990	3.95
1991	4.04
1992	4.13
1993	4.22
1994	4.32
1995	4.41
1996	4.51
1997	4.61
1998	4.71
1999	4.81
2000	4.92
2001	5.02
2002	5.13
2003	5.23
2004	5.34
2005	5.45
2006	5.57
2007	5.68
2008	5.79
2009	5.91
2010	6.03
2011	6.15
2012	6.27
2013	6.39
2014	6.51
2015	6.63