Year	Years
1970	3.6
1971	3.68
1972	3.77
1973	3.86
1974	3.94
1975	4.03
1976	4.13
1977	4.22
1978	4.31
1979	4.41
1980	4.51
1981	4.6
1982	4.71
1983	4.81
1984	4.91
1985	5.02
1986	5.12
1987	5.23
1988	5.34
1989	5.45
1990	5.57
1991	5.68
1992	5.8
1993	5.91
1994	6.03
1995	6.15
1996	6.27
1997	6.39
1998	6.52
1999	6.64
2000	6.76
2001	6.89
2002	7.02
2003	7.15
2004	7.28
2005	7.41
2006	7.54
2007	7.67
2008	7.81
2009	7.94
2010	8.08
2011	8.22
2012	8.35
2013	8.49
2014	8.63
2015	8.77