Year	Years
1970	2.67
1971	2.75
1972	2.83
1973	2.92
1974	3
1975	3.09
1976	3.18
1977	3.27
1978	3.37
1979	3.46
1980	3.56
1981	3.65
1982	3.75
1983	3.85
1984	3.95
1985	4.06
1986	4.16
1987	4.26
1988	4.37
1989	4.47
1990	4.58
1991	4.68
1992	4.79
1993	4.89
1994	5
1995	5.1
1996	5.21
1997	5.31
1998	5.41
1999	5.52
2000	5.63
2001	5.74
2002	5.85
2003	5.96
2004	6.08
2005	6.2
2006	6.32
2007	6.45
2008	6.57
2009	6.7
2010	6.84
2011	6.98
2012	7.12
2013	7.26
2014	7.4
2015	7.55