Year	Years
1970	3.46
1971	3.56
1972	3.66
1973	3.77
1974	3.87
1975	3.98
1976	4.09
1977	4.2
1978	4.32
1979	4.43
1980	4.55
1981	4.67
1982	4.8
1983	4.92
1984	5.05
1985	5.18
1986	5.32
1987	5.45
1988	5.59
1989	5.73
1990	5.87
1991	6.01
1992	6.16
1993	6.3
1994	6.45
1995	6.6
1996	6.76
1997	6.91
1998	7.06
1999	7.22
2000	7.38
2001	7.53
2002	7.69
2003	7.86
2004	8.02
2005	8.18
2006	8.35
2007	8.51
2008	8.68
2009	8.84
2010	9.01
2011	9.18
2012	9.35
2013	9.51
2014	9.68
2015	9.85