Year	Years
1970	3.53
1971	3.63
1972	3.73
1973	3.83
1974	3.93
1975	4.04
1976	4.15
1977	4.26
1978	4.37
1979	4.48
1980	4.59
1981	4.71
1982	4.83
1983	4.95
1984	5.07
1985	5.2
1986	5.32
1987	5.45
1988	5.58
1989	5.71
1990	5.84
1991	5.97
1992	6.11
1993	6.24
1994	6.38
1995	6.52
1996	6.66
1997	6.8
1998	6.94
1999	7.09
2000	7.23
2001	7.38
2002	7.53
2003	7.68
2004	7.84
2005	7.99
2006	8.15
2007	8.3
2008	8.46
2009	8.62
2010	8.78
2011	8.95
2012	9.11
2013	9.28
2014	9.44
2015	9.61