Year	Years
1970	3.06
1971	3.15
1972	3.24
1973	3.34
1974	3.44
1975	3.54
1976	3.65
1977	3.75
1978	3.86
1979	3.97
1980	4.09
1981	4.2
1982	4.32
1983	4.44
1984	4.56
1985	4.68
1986	4.8
1987	4.93
1988	5.05
1989	5.18
1990	5.31
1991	5.44
1992	5.57
1993	5.7
1994	5.83
1995	5.96
1996	6.09
1997	6.23
1998	6.36
1999	6.5
2000	6.63
2001	6.77
2002	6.91
2003	7.05
2004	7.19
2005	7.33
2006	7.48
2007	7.62
2008	7.77
2009	7.91
2010	8.06
2011	8.21
2012	8.36
2013	8.51
2014	8.67
2015	8.82