Year	Years
1970	3.96
1971	4.05
1972	4.14
1973	4.24
1974	4.33
1975	4.42
1976	4.52
1977	4.61
1978	4.71
1979	4.81
1980	4.91
1981	5.01
1982	5.11
1983	5.21
1984	5.31
1985	5.42
1986	5.52
1987	5.63
1988	5.73
1989	5.84
1990	5.94
1991	6.05
1992	6.16
1993	6.27
1994	6.38
1995	6.49
1996	6.6
1997	6.71
1998	6.83
1999	6.95
2000	7.07
2001	7.18
2002	7.31
2003	7.43
2004	7.55
2005	7.68
2006	7.8
2007	7.93
2008	8.06
2009	8.18
2010	8.31
2011	8.44
2012	8.58
2013	8.71
2014	8.84
2015	8.97