Year	Years
1970	2.52
1971	2.59
1972	2.67
1973	2.75
1974	2.83
1975	2.91
1976	3
1977	3.08
1978	3.17
1979	3.26
1980	3.35
1981	3.45
1982	3.54
1983	3.64
1984	3.74
1985	3.84
1986	3.94
1987	4.05
1988	4.16
1989	4.26
1990	4.37
1991	4.49
1992	4.6
1993	4.71
1994	4.83
1995	4.95
1996	5.07
1997	5.19
1998	5.31
1999	5.44
2000	5.57
2001	5.7
2002	5.83
2003	5.96
2004	6.1
2005	6.24
2006	6.38
2007	6.52
2008	6.67
2009	6.82
2010	6.97
2011	7.12
2012	7.27
2013	7.42
2014	7.58
2015	7.74