Year	Years
1970	3.91
1971	4.02
1972	4.13
1973	4.24
1974	4.35
1975	4.46
1976	4.58
1977	4.69
1978	4.81
1979	4.94
1980	5.06
1981	5.18
1982	5.31
1983	5.44
1984	5.57
1985	5.7
1986	5.84
1987	5.97
1988	6.11
1989	6.25
1990	6.39
1991	6.53
1992	6.67
1993	6.82
1994	6.96
1995	7.11
1996	7.26
1997	7.41
1998	7.57
1999	7.72
2000	7.88
2001	8.04
2002	8.2
2003	8.36
2004	8.52
2005	8.69
2006	8.85
2007	9.02
2008	9.19
2009	9.35
2010	9.52
2011	9.69
2012	9.86
2013	10.03
2014	10.2
2015	10.37