Year	Years
1970	4.24
1971	4.33
1972	4.43
1973	4.53
1974	4.63
1975	4.73
1976	4.84
1977	4.94
1978	5.05
1979	5.16
1980	5.27
1981	5.38
1982	5.49
1983	5.6
1984	5.72
1985	5.83
1986	5.95
1987	6.07
1988	6.19
1989	6.31
1990	6.44
1991	6.56
1992	6.69
1993	6.81
1994	6.94
1995	7.07
1996	7.2
1997	7.33
1998	7.46
1999	7.59
2000	7.73
2001	7.86
2002	7.99
2003	8.13
2004	8.26
2005	8.4
2006	8.53
2007	8.67
2008	8.81
2009	8.94
2010	9.08
2011	9.22
2012	9.36
2013	9.5
2014	9.63
2015	9.77