Year	Years
1970	2.53
1971	2.6
1972	2.68
1973	2.75
1974	2.83
1975	2.91
1976	3
1977	3.08
1978	3.16
1979	3.25
1980	3.34
1981	3.43
1982	3.52
1983	3.62
1984	3.71
1985	3.81
1986	3.91
1987	4
1988	4.11
1989	4.21
1990	4.31
1991	4.42
1992	4.52
1993	4.63
1994	4.74
1995	4.85
1996	4.97
1997	5.08
1998	5.2
1999	5.32
2000	5.45
2001	5.58
2002	5.71
2003	5.84
2004	5.97
2005	6.11
2006	6.25
2007	6.39
2008	6.54
2009	6.69
2010	6.83
2011	6.98
2012	7.14
2013	7.29
2014	7.45
2015	7.6