Year	Years
1970	2.56
1971	2.63
1972	2.71
1973	2.79
1974	2.87
1975	2.95
1976	3.04
1977	3.13
1978	3.22
1979	3.31
1980	3.4
1981	3.5
1982	3.6
1983	3.7
1984	3.8
1985	3.9
1986	4.01
1987	4.12
1988	4.23
1989	4.34
1990	4.46
1991	4.57
1992	4.69
1993	4.82
1994	4.94
1995	5.07
1996	5.2
1997	5.33
1998	5.46
1999	5.59
2000	5.73
2001	5.87
2002	6.01
2003	6.15
2004	6.3
2005	6.45
2006	6.59
2007	6.74
2008	6.9
2009	7.05
2010	7.2
2011	7.36
2012	7.52
2013	7.68
2014	7.84
2015	8