Year	Years
1970	3.61
1971	3.71
1972	3.82
1973	3.92
1974	4.03
1975	4.14
1976	4.25
1977	4.36
1978	4.47
1979	4.59
1980	4.71
1981	4.83
1982	4.95
1983	5.07
1984	5.19
1985	5.32
1986	5.45
1987	5.57
1988	5.7
1989	5.83
1990	5.96
1991	6.1
1992	6.23
1993	6.36
1994	6.5
1995	6.63
1996	6.77
1997	6.9
1998	7.04
1999	7.18
2000	7.32
2001	7.46
2002	7.61
2003	7.76
2004	7.91
2005	8.06
2006	8.21
2007	8.36
2008	8.52
2009	8.68
2010	8.84
2011	9
2012	9.16
2013	9.32
2014	9.49
2015	9.65