Year	Years
1970	2.42
1971	2.49
1972	2.55
1973	2.62
1974	2.68
1975	2.75
1976	2.82
1977	2.9
1978	2.97
1979	3.05
1980	3.12
1981	3.2
1982	3.29
1983	3.37
1984	3.45
1985	3.54
1986	3.63
1987	3.72
1988	3.81
1989	3.91
1990	4
1991	4.1
1992	4.2
1993	4.31
1994	4.41
1995	4.52
1996	4.63
1997	4.74
1998	4.85
1999	4.97
2000	5.08
2001	5.19
2002	5.31
2003	5.42
2004	5.54
2005	5.66
2006	5.77
2007	5.89
2008	6.01
2009	6.13
2010	6.26
2011	6.38
2012	6.5
2013	6.63
2014	6.75
2015	6.88