Year	Years
1970	4.06
1971	4.16
1972	4.27
1973	4.38
1974	4.49
1975	4.61
1976	4.72
1977	4.84
1978	4.96
1979	5.08
1980	5.2
1981	5.32
1982	5.45
1983	5.57
1984	5.7
1985	5.83
1986	5.96
1987	6.09
1988	6.22
1989	6.35
1990	6.49
1991	6.62
1992	6.76
1993	6.9
1994	7.04
1995	7.18
1996	7.33
1997	7.49
1998	7.65
1999	7.8
2000	7.96
2001	8.12
2002	8.28
2003	8.45
2004	8.61
2005	8.78
2006	8.94
2007	9.11
2008	9.27
2009	9.44
2010	9.6
2011	9.77
2012	9.94
2013	10.1
2014	10.27
2015	10.44