Year	Years
1970	3.09
1971	3.19
1972	3.28
1973	3.38
1974	3.48
1975	3.59
1976	3.7
1977	3.81
1978	3.92
1979	4.04
1980	4.16
1981	4.28
1982	4.4
1983	4.53
1984	4.66
1985	4.79
1986	4.93
1987	5.06
1988	5.2
1989	5.35
1990	5.49
1991	5.64
1992	5.79
1993	5.94
1994	6.09
1995	6.25
1996	6.4
1997	6.56
1998	6.71
1999	6.87
2000	7.03
2001	7.19
2002	7.35
2003	7.51
2004	7.67
2005	7.83
2006	7.99
2007	8.15
2008	8.31
2009	8.48
2010	8.64
2011	8.8
2012	8.96
2013	9.12
2014	9.28
2015	9.44