Year	Years
1970	3.49
1971	3.59
1972	3.69
1973	3.79
1974	3.9
1975	4
1976	4.11
1977	4.22
1978	4.33
1979	4.44
1980	4.55
1981	4.67
1982	4.78
1983	4.9
1984	5.02
1985	5.14
1986	5.26
1987	5.38
1988	5.5
1989	5.62
1990	5.74
1991	5.86
1992	5.97
1993	6.09
1994	6.2
1995	6.31
1996	6.44
1997	6.57
1998	6.71
1999	6.85
2000	6.99
2001	7.13
2002	7.28
2003	7.43
2004	7.59
2005	7.74
2006	7.9
2007	8.06
2008	8.22
2009	8.38
2010	8.54
2011	8.71
2012	8.87
2013	9.04
2014	9.2
2015	9.37