Year	Years
1970	3.57
1971	3.67
1972	3.77
1973	3.88
1974	3.98
1975	4.09
1976	4.2
1977	4.32
1978	4.43
1979	4.55
1980	4.67
1981	4.79
1982	4.92
1983	5.04
1984	5.17
1985	5.3
1986	5.43
1987	5.57
1988	5.7
1989	5.84
1990	5.98
1991	6.12
1992	6.26
1993	6.4
1994	6.54
1995	6.69
1996	6.83
1997	6.97
1998	7.12
1999	7.27
2000	7.42
2001	7.57
2002	7.72
2003	7.88
2004	8.03
2005	8.19
2006	8.35
2007	8.51
2008	8.67
2009	8.83
2010	8.99
2011	9.16
2012	9.32
2013	9.49
2014	9.65
2015	9.82