Year	Years
1970	2.84
1971	2.92
1972	3
1973	3.08
1974	3.16
1975	3.25
1976	3.34
1977	3.43
1978	3.52
1979	3.62
1980	3.72
1981	3.82
1982	3.93
1983	4.04
1984	4.15
1985	4.26
1986	4.38
1987	4.5
1988	4.62
1989	4.74
1990	4.87
1991	5
1992	5.14
1993	5.28
1994	5.41
1995	5.56
1996	5.69
1997	5.83
1998	5.97
1999	6.1
2000	6.24
2001	6.38
2002	6.51
2003	6.65
2004	6.78
2005	6.91
2006	7.05
2007	7.18
2008	7.31
2009	7.44
2010	7.58
2011	7.71
2012	7.84
2013	7.97
2014	8.1
2015	8.23