Year	Years
1970	3.04
1971	3.13
1972	3.21
1973	3.3
1974	3.4
1975	3.49
1976	3.59
1977	3.69
1978	3.79
1979	3.89
1980	3.99
1981	4.1
1982	4.21
1983	4.32
1984	4.43
1985	4.55
1986	4.67
1987	4.79
1988	4.91
1989	5.03
1990	5.16
1991	5.29
1992	5.42
1993	5.55
1994	5.69
1995	5.83
1996	5.97
1997	6.12
1998	6.27
1999	6.42
2000	6.57
2001	6.72
2002	6.88
2003	7.03
2004	7.19
2005	7.35
2006	7.51
2007	7.67
2008	7.84
2009	8
2010	8.17
2011	8.33
2012	8.5
2013	8.67
2014	8.84
2015	9.01