Year	Years
1970	3.9
1971	3.99
1972	4.09
1973	4.18
1974	4.28
1975	4.38
1976	4.48
1977	4.59
1978	4.69
1979	4.8
1980	4.91
1981	5.02
1982	5.13
1983	5.24
1984	5.36
1985	5.48
1986	5.6
1987	5.72
1988	5.84
1989	5.97
1990	6.1
1991	6.23
1992	6.36
1993	6.49
1994	6.62
1995	6.76
1996	6.9
1997	7.04
1998	7.18
1999	7.32
2000	7.46
2001	7.6
2002	7.74
2003	7.88
2004	8.02
2005	8.16
2006	8.3
2007	8.44
2008	8.58
2009	8.72
2010	8.86
2011	9
2012	9.14
2013	9.28
2014	9.41
2015	9.55