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	3.06
1979	3.13
1980	3.21
1981	3.28
1982	3.36
1983	3.45
1984	3.53
1985	3.61
1986	3.7
1987	3.79
1988	3.88
1989	3.97
1990	4.07
1991	4.16
1992	4.26
1993	4.36
1994	4.46
1995	4.57
1996	4.68
1997	4.79
1998	4.9
1999	5.01
2000	5.13
2001	5.24
2002	5.36
2003	5.48
2004	5.59
2005	5.71
2006	5.83
2007	5.95
2008	6.08
2009	6.2
2010	6.32
2011	6.45
2012	6.58
2013	6.7
2014	6.83
2015	6.96