Year	Years
1970	3.16
1971	3.26
1972	3.35
1973	3.45
1974	3.54
1975	3.64
1976	3.75
1977	3.85
1978	3.96
1979	4.06
1980	4.18
1981	4.29
1982	4.4
1983	4.52
1984	4.64
1985	4.76
1986	4.88
1987	5
1988	5.13
1989	5.26
1990	5.38
1991	5.51
1992	5.65
1993	5.78
1994	5.91
1995	6.05
1996	6.18
1997	6.32
1998	6.47
1999	6.61
2000	6.75
2001	6.9
2002	7.05
2003	7.2
2004	7.35
2005	7.5
2006	7.65
2007	7.81
2008	7.96
2009	8.12
2010	8.28
2011	8.44
2012	8.59
2013	8.76
2014	8.92
2015	9.08