Year	Years
1970	3.78
1971	3.87
1972	3.96
1973	4.06
1974	4.15
1975	4.24
1976	4.34
1977	4.44
1978	4.54
1979	4.64
1980	4.74
1981	4.84
1982	4.95
1983	5.05
1984	5.16
1985	5.26
1986	5.37
1987	5.48
1988	5.59
1989	5.7
1990	5.81
1991	5.92
1992	6.03
1993	6.14
1994	6.25
1995	6.36
1996	6.46
1997	6.57
1998	6.68
1999	6.8
2000	6.91
2001	7.02
2002	7.14
2003	7.25
2004	7.37
2005	7.49
2006	7.61
2007	7.73
2008	7.85
2009	7.98
2010	8.1
2011	8.23
2012	8.35
2013	8.48
2014	8.61
2015	8.74