Year	Years
1970	2.41
1971	2.48
1972	2.54
1973	2.61
1974	2.68
1975	2.75
1976	2.82
1977	2.9
1978	2.97
1979	3.05
1980	3.13
1981	3.21
1982	3.29
1983	3.38
1984	3.46
1985	3.55
1986	3.63
1987	3.72
1988	3.81
1989	3.9
1990	4
1991	4.09
1992	4.18
1993	4.28
1994	4.37
1995	4.46
1996	4.54
1997	4.63
1998	4.71
1999	4.8
2000	4.89
2001	4.97
2002	5.06
2003	5.16
2004	5.25
2005	5.34
2006	5.44
2007	5.53
2008	5.63
2009	5.73
2010	5.83
2011	5.93
2012	6.04
2013	6.14
2014	6.25
2015	6.35