Year	Years
1970	3.71
1971	3.8
1972	3.89
1973	3.98
1974	4.07
1975	4.16
1976	4.26
1977	4.36
1978	4.45
1979	4.55
1980	4.65
1981	4.76
1982	4.86
1983	4.96
1984	5.07
1985	5.17
1986	5.28
1987	5.38
1988	5.49
1989	5.6
1990	5.71
1991	5.81
1992	5.92
1993	6.03
1994	6.14
1995	6.24
1996	6.35
1997	6.45
1998	6.56
1999	6.67
2000	6.77
2001	6.88
2002	6.99
2003	7.1
2004	7.22
2005	7.33
2006	7.45
2007	7.56
2008	7.68
2009	7.8
2010	7.92
2011	8.05
2012	8.17
2013	8.3
2014	8.43
2015	8.56