Year	Years
1970	2.51
1971	2.57
1972	2.64
1973	2.7
1974	2.77
1975	2.84
1976	2.91
1977	2.98
1978	3.05
1979	3.13
1980	3.2
1981	3.28
1982	3.36
1983	3.44
1984	3.53
1985	3.61
1986	3.7
1987	3.78
1988	3.87
1989	3.96
1990	4.05
1991	4.15
1992	4.24
1993	4.34
1994	4.44
1995	4.54
1996	4.64
1997	4.74
1998	4.85
1999	4.95
2000	5.06
2001	5.17
2002	5.28
2003	5.39
2004	5.51
2005	5.62
2006	5.74
2007	5.86
2008	5.97
2009	6.09
2010	6.21
2011	6.34
2012	6.46
2013	6.58
2014	6.71
2015	6.83