Year	Years
1970	2.24
1971	2.31
1972	2.38
1973	2.45
1974	2.52
1975	2.6
1976	2.67
1977	2.75
1978	2.83
1979	2.91
1980	2.99
1981	3.08
1982	3.16
1983	3.25
1984	3.34
1985	3.43
1986	3.53
1987	3.63
1988	3.72
1989	3.83
1990	3.93
1991	4.03
1992	4.14
1993	4.25
1994	4.36
1995	4.47
1996	4.59
1997	4.71
1998	4.84
1999	4.96
2000	5.09
2001	5.22
2002	5.36
2003	5.49
2004	5.63
2005	5.77
2006	5.91
2007	6.05
2008	6.2
2009	6.35
2010	6.5
2011	6.65
2012	6.8
2013	6.96
2014	7.11
2015	7.27