Year	Years
1970	3.46
1971	3.55
1972	3.65
1973	3.75
1974	3.85
1975	3.95
1976	4.06
1977	4.17
1978	4.28
1979	4.4
1980	4.51
1981	4.64
1982	4.76
1983	4.89
1984	5.02
1985	5.15
1986	5.29
1987	5.43
1988	5.58
1989	5.72
1990	5.88
1991	6.03
1992	6.19
1993	6.35
1994	6.52
1995	6.68
1996	6.86
1997	7.03
1998	7.2
1999	7.37
2000	7.54
2001	7.7
2002	7.87
2003	8.04
2004	8.2
2005	8.36
2006	8.52
2007	8.68
2008	8.84
2009	8.99
2010	9.14
2011	9.29
2012	9.44
2013	9.59
2014	9.73
2015	9.87