Year	Years
1970	3.88
1971	3.97
1972	4.06
1973	4.15
1974	4.25
1975	4.34
1976	4.44
1977	4.54
1978	4.64
1979	4.74
1980	4.84
1981	4.94
1982	5.05
1983	5.15
1984	5.26
1985	5.37
1986	5.48
1987	5.59
1988	5.71
1989	5.82
1990	5.94
1991	6.06
1992	6.18
1993	6.3
1994	6.42
1995	6.54
1996	6.66
1997	6.79
1998	6.91
1999	7.04
2000	7.17
2001	7.3
2002	7.43
2003	7.56
2004	7.69
2005	7.82
2006	7.96
2007	8.09
2008	8.23
2009	8.36
2010	8.5
2011	8.63
2012	8.77
2013	8.91
2014	9.05
2015	9.18