Year	Years
1970	3.54
1971	3.64
1972	3.74
1973	3.84
1974	3.94
1975	4.05
1976	4.16
1977	4.27
1978	4.39
1979	4.5
1980	4.62
1981	4.74
1982	4.86
1983	4.99
1984	5.11
1985	5.24
1986	5.37
1987	5.5
1988	5.64
1989	5.77
1990	5.91
1991	6.05
1992	6.19
1993	6.33
1994	6.48
1995	6.63
1996	6.77
1997	6.91
1998	7.06
1999	7.21
2000	7.36
2001	7.51
2002	7.67
2003	7.82
2004	7.98
2005	8.14
2006	8.3
2007	8.46
2008	8.62
2009	8.78
2010	8.95
2011	9.11
2012	9.28
2013	9.44
2014	9.61
2015	9.78