Year	Years
1970	2.27
1971	2.32
1972	2.37
1973	2.42
1974	2.48
1975	2.53
1976	2.58
1977	2.64
1978	2.69
1979	2.75
1980	2.8
1981	2.86
1982	2.92
1983	2.97
1984	3.03
1985	3.08
1986	3.14
1987	3.19
1988	3.24
1989	3.29
1990	3.34
1991	3.39
1992	3.44
1993	3.48
1994	3.52
1995	3.56
1996	3.6
1997	3.64
1998	3.69
1999	3.74
2000	3.8
2001	3.86
2002	3.92
2003	3.98
2004	4.05
2005	4.12
2006	4.19
2007	4.27
2008	4.35
2009	4.43
2010	4.51
2011	4.6
2012	4.68
2013	4.77
2014	4.86
2015	4.95