Year	Years
1970	3.9
1971	3.99
1972	4.08
1973	4.17
1974	4.26
1975	4.36
1976	4.45
1977	4.55
1978	4.65
1979	4.75
1980	4.84
1981	4.95
1982	5.05
1983	5.15
1984	5.25
1985	5.36
1986	5.47
1987	5.57
1988	5.68
1989	5.79
1990	5.9
1991	6.01
1992	6.12
1993	6.23
1994	6.34
1995	6.45
1996	6.57
1997	6.68
1998	6.8
1999	6.92
2000	7.04
2001	7.16
2002	7.28
2003	7.4
2004	7.53
2005	7.65
2006	7.78
2007	7.91
2008	8.04
2009	8.17
2010	8.3
2011	8.43
2012	8.57
2013	8.7
2014	8.83
2015	8.97