Year	Years
1970	2.92
1971	3
1972	3.09
1973	3.18
1974	3.27
1975	3.36
1976	3.45
1977	3.55
1978	3.65
1979	3.75
1980	3.85
1981	3.96
1982	4.07
1983	4.18
1984	4.29
1985	4.4
1986	4.52
1987	4.63
1988	4.75
1989	4.88
1990	5
1991	5.13
1992	5.25
1993	5.38
1994	5.52
1995	5.65
1996	5.79
1997	5.93
1998	6.08
1999	6.22
2000	6.37
2001	6.52
2002	6.68
2003	6.83
2004	6.99
2005	7.15
2006	7.31
2007	7.47
2008	7.63
2009	7.8
2010	7.96
2011	8.13
2012	8.3
2013	8.46
2014	8.63
2015	8.8