Year	Years
1970	3.54
1971	3.63
1972	3.71
1973	3.8
1974	3.89
1975	3.98
1976	4.07
1977	4.16
1978	4.26
1979	4.35
1980	4.45
1981	4.55
1982	4.65
1983	4.75
1984	4.86
1985	4.96
1986	5.07
1987	5.18
1988	5.29
1989	5.4
1990	5.52
1991	5.63
1992	5.75
1993	5.87
1994	5.99
1995	6.11
1996	6.23
1997	6.35
1998	6.47
1999	6.59
2000	6.72
2001	6.85
2002	6.97
2003	7.1
2004	7.23
2005	7.36
2006	7.49
2007	7.63
2008	7.76
2009	7.89
2010	8.03
2011	8.16
2012	8.3
2013	8.43
2014	8.57
2015	8.71