Year	Years
1970	3.93
1971	4.03
1972	4.13
1973	4.24
1974	4.34
1975	4.45
1976	4.57
1977	4.68
1978	4.8
1979	4.92
1980	5.05
1981	5.18
1982	5.31
1983	5.45
1984	5.59
1985	5.73
1986	5.87
1987	6.02
1988	6.18
1989	6.34
1990	6.5
1991	6.66
1992	6.83
1993	7
1994	7.18
1995	7.36
1996	7.54
1997	7.71
1998	7.89
1999	8.06
2000	8.22
2001	8.39
2002	8.55
2003	8.71
2004	8.87
2005	9.03
2006	9.18
2007	9.34
2008	9.49
2009	9.64
2010	9.79
2011	9.93
2012	10.08
2013	10.22
2014	10.36
2015	10.51