Year	Years
1970	4.14
1971	4.24
1972	4.33
1973	4.43
1974	4.53
1975	4.63
1976	4.73
1977	4.83
1978	4.94
1979	5.05
1980	5.15
1981	5.26
1982	5.38
1983	5.49
1984	5.6
1985	5.72
1986	5.84
1987	5.96
1988	6.08
1989	6.2
1990	6.32
1991	6.45
1992	6.57
1993	6.7
1994	6.83
1995	6.96
1996	7.08
1997	7.21
1998	7.34
1999	7.48
2000	7.61
2001	7.74
2002	7.87
2003	8.01
2004	8.14
2005	8.28
2006	8.42
2007	8.56
2008	8.69
2009	8.83
2010	8.97
2011	9.11
2012	9.25
2013	9.39
2014	9.53
2015	9.67