Year	Years
1970	3.33
1971	3.42
1972	3.52
1973	3.62
1974	3.72
1975	3.83
1976	3.93
1977	4.04
1978	4.15
1979	4.27
1980	4.38
1981	4.5
1982	4.62
1983	4.75
1984	4.87
1985	5
1986	5.13
1987	5.26
1988	5.39
1989	5.53
1990	5.67
1991	5.81
1992	5.95
1993	6.1
1994	6.24
1995	6.39
1996	6.55
1997	6.71
1998	6.86
1999	7.02
2000	7.19
2001	7.35
2002	7.51
2003	7.68
2004	7.84
2005	8.01
2006	8.18
2007	8.35
2008	8.52
2009	8.69
2010	8.86
2011	9.03
2012	9.2
2013	9.37
2014	9.54
2015	9.71