Year	Years
1970	3.39
1971	3.49
1972	3.59
1973	3.69
1974	3.79
1975	3.89
1976	4
1977	4.1
1978	4.21
1979	4.32
1980	4.44
1981	4.55
1982	4.67
1983	4.79
1984	4.91
1985	5.03
1986	5.15
1987	5.28
1988	5.4
1989	5.53
1990	5.66
1991	5.79
1992	5.92
1993	6.05
1994	6.18
1995	6.31
1996	6.45
1997	6.59
1998	6.73
1999	6.87
2000	7.01
2001	7.16
2002	7.3
2003	7.45
2004	7.6
2005	7.75
2006	7.91
2007	8.06
2008	8.22
2009	8.37
2010	8.53
2011	8.69
2012	8.85
2013	9.01
2014	9.18
2015	9.34