Year	Years
1970	2.38
1971	2.44
1972	2.5
1973	2.56
1974	2.62
1975	2.68
1976	2.75
1977	2.82
1978	2.88
1979	2.95
1980	3.02
1981	3.1
1982	3.17
1983	3.24
1984	3.32
1985	3.4
1986	3.48
1987	3.56
1988	3.64
1989	3.72
1990	3.81
1991	3.89
1992	3.98
1993	4.07
1994	4.16
1995	4.25
1996	4.35
1997	4.44
1998	4.54
1999	4.64
2000	4.74
2001	4.84
2002	4.94
2003	5.05
2004	5.15
2005	5.26
2006	5.37
2007	5.48
2008	5.59
2009	5.71
2010	5.82
2011	5.94
2012	6.05
2013	6.17
2014	6.29
2015	6.41