Year	Years
1970	2.96
1971	3.05
1972	3.14
1973	3.23
1974	3.32
1975	3.41
1976	3.51
1977	3.61
1978	3.7
1979	3.81
1980	3.91
1981	4.01
1982	4.12
1983	4.22
1984	4.33
1985	4.44
1986	4.55
1987	4.67
1988	4.78
1989	4.89
1990	5.01
1991	5.13
1992	5.24
1993	5.36
1994	5.48
1995	5.6
1996	5.72
1997	5.84
1998	5.97
1999	6.09
2000	6.22
2001	6.36
2002	6.49
2003	6.63
2004	6.76
2005	6.9
2006	7.04
2007	7.19
2008	7.33
2009	7.48
2010	7.63
2011	7.78
2012	7.93
2013	8.08
2014	8.24
2015	8.39