Year	Years
1970	3.9
1971	3.99
1972	4.08
1973	4.17
1974	4.26
1975	4.36
1976	4.45
1977	4.55
1978	4.65
1979	4.75
1980	4.85
1981	4.95
1982	5.06
1983	5.16
1984	5.27
1985	5.38
1986	5.49
1987	5.6
1988	5.71
1989	5.83
1990	5.94
1991	6.06
1992	6.18
1993	6.29
1994	6.42
1995	6.54
1996	6.66
1997	6.78
1998	6.91
1999	7.03
2000	7.16
2001	7.29
2002	7.42
2003	7.55
2004	7.68
2005	7.81
2006	7.95
2007	8.08
2008	8.22
2009	8.35
2010	8.49
2011	8.63
2012	8.76
2013	8.9
2014	9.04
2015	9.18