Year	Years
1970	2.26
1971	2.32
1972	2.38
1973	2.44
1974	2.5
1975	2.56
1976	2.63
1977	2.7
1978	2.76
1979	2.83
1980	2.91
1981	2.98
1982	3.05
1983	3.13
1984	3.21
1985	3.29
1986	3.37
1987	3.45
1988	3.54
1989	3.62
1990	3.71
1991	3.81
1992	3.9
1993	3.99
1994	4.09
1995	4.19
1996	4.3
1997	4.41
1998	4.52
1999	4.63
2000	4.74
2001	4.85
2002	4.97
2003	5.08
2004	5.2
2005	5.32
2006	5.44
2007	5.56
2008	5.68
2009	5.8
2010	5.92
2011	6.05
2012	6.17
2013	6.3
2014	6.42
2015	6.55