Year	Years
1970	4.22
1971	4.34
1972	4.46
1973	4.58
1974	4.7
1975	4.83
1976	4.95
1977	5.08
1978	5.22
1979	5.35
1980	5.49
1981	5.63
1982	5.77
1983	5.91
1984	6.06
1985	6.21
1986	6.36
1987	6.51
1988	6.67
1989	6.83
1990	6.99
1991	7.15
1992	7.31
1993	7.48
1994	7.65
1995	7.82
1996	7.99
1997	8.16
1998	8.34
1999	8.51
2000	8.69
2001	8.86
2002	9.04
2003	9.22
2004	9.39
2005	9.57
2006	9.74
2007	9.92
2008	10.09
2009	10.26
2010	10.44
2011	10.61
2012	10.78
2013	10.95
2014	11.12
2015	11.29