Year	Years
1970	3.35
1971	3.45
1972	3.54
1973	3.64
1974	3.75
1975	3.85
1976	3.96
1977	4.06
1978	4.17
1979	4.29
1980	4.4
1981	4.52
1982	4.64
1983	4.76
1984	4.88
1985	5.01
1986	5.13
1987	5.26
1988	5.4
1989	5.53
1990	5.66
1991	5.8
1992	5.94
1993	6.08
1994	6.22
1995	6.37
1996	6.51
1997	6.65
1998	6.8
1999	6.94
2000	7.09
2001	7.24
2002	7.4
2003	7.55
2004	7.7
2005	7.86
2006	8.02
2007	8.18
2008	8.34
2009	8.5
2010	8.66
2011	8.82
2012	8.99
2013	9.15
2014	9.31
2015	9.48