Year	Years
1970	3.59
1971	3.68
1972	3.76
1973	3.85
1974	3.94
1975	4.02
1976	4.11
1977	4.21
1978	4.3
1979	4.4
1980	4.49
1981	4.59
1982	4.69
1983	4.79
1984	4.89
1985	5
1986	5.11
1987	5.21
1988	5.32
1989	5.44
1990	5.55
1991	5.66
1992	5.78
1993	5.9
1994	6.02
1995	6.14
1996	6.27
1997	6.4
1998	6.53
1999	6.67
2000	6.8
2001	6.94
2002	7.07
2003	7.21
2004	7.35
2005	7.49
2006	7.63
2007	7.77
2008	7.91
2009	8.05
2010	8.2
2011	8.34
2012	8.48
2013	8.63
2014	8.77
2015	8.92