Year	Years
1970	4.48
1971	4.58
1972	4.69
1973	4.79
1974	4.89
1975	5
1976	5.1
1977	5.21
1978	5.32
1979	5.43
1980	5.54
1981	5.65
1982	5.77
1983	5.88
1984	5.99
1985	6.11
1986	6.22
1987	6.34
1988	6.46
1989	6.57
1990	6.69
1991	6.81
1992	6.92
1993	7.04
1994	7.16
1995	7.27
1996	7.39
1997	7.51
1998	7.62
1999	7.74
2000	7.86
2001	7.98
2002	8.1
2003	8.22
2004	8.34
2005	8.46
2006	8.59
2007	8.71
2008	8.84
2009	8.97
2010	9.1
2011	9.22
2012	9.36
2013	9.49
2014	9.62
2015	9.75