Year	Years
1970	4.09
1971	4.18
1972	4.27
1973	4.36
1974	4.46
1975	4.55
1976	4.65
1977	4.75
1978	4.85
1979	4.95
1980	5.05
1981	5.15
1982	5.25
1983	5.36
1984	5.46
1985	5.57
1986	5.68
1987	5.79
1988	5.9
1989	6.01
1990	6.12
1991	6.24
1992	6.35
1993	6.47
1994	6.59
1995	6.7
1996	6.82
1997	6.95
1998	7.07
1999	7.19
2000	7.32
2001	7.45
2002	7.57
2003	7.7
2004	7.84
2005	7.97
2006	8.1
2007	8.23
2008	8.37
2009	8.51
2010	8.64
2011	8.78
2012	8.92
2013	9.06
2014	9.2
2015	9.34