Year	Years
1970	3.71
1971	3.81
1972	3.92
1973	4.03
1974	4.14
1975	4.25
1976	4.36
1977	4.48
1978	4.59
1979	4.71
1980	4.84
1981	4.96
1982	5.09
1983	5.22
1984	5.35
1985	5.48
1986	5.61
1987	5.75
1988	5.89
1989	6.03
1990	6.17
1991	6.31
1992	6.46
1993	6.6
1994	6.75
1995	6.9
1996	7.05
1997	7.21
1998	7.36
1999	7.52
2000	7.68
2001	7.84
2002	8
2003	8.16
2004	8.32
2005	8.48
2006	8.65
2007	8.81
2008	8.98
2009	9.15
2010	9.31
2011	9.48
2012	9.65
2013	9.82
2014	9.99
2015	10.16