Year	Years
1970	4.48
1971	4.57
1972	4.67
1973	4.77
1974	4.88
1975	4.98
1976	5.08
1977	5.19
1978	5.3
1979	5.41
1980	5.52
1981	5.63
1982	5.74
1983	5.85
1984	5.97
1985	6.09
1986	6.2
1987	6.32
1988	6.44
1989	6.56
1990	6.69
1991	6.81
1992	6.93
1993	7.06
1994	7.19
1995	7.31
1996	7.44
1997	7.58
1998	7.71
1999	7.84
2000	7.98
2001	8.11
2002	8.25
2003	8.38
2004	8.52
2005	8.66
2006	8.8
2007	8.94
2008	9.07
2009	9.21
2010	9.36
2011	9.5
2012	9.64
2013	9.78
2014	9.92
2015	10.06