Year	Years
1970	4.34
1971	4.44
1972	4.54
1973	4.64
1974	4.74
1975	4.84
1976	4.95
1977	5.05
1978	5.16
1979	5.27
1980	5.38
1981	5.49
1982	5.6
1983	5.72
1984	5.83
1985	5.95
1986	6.07
1987	6.19
1988	6.31
1989	6.43
1990	6.56
1991	6.68
1992	6.81
1993	6.93
1994	7.06
1995	7.19
1996	7.32
1997	7.45
1998	7.59
1999	7.72
2000	7.85
2001	7.99
2002	8.12
2003	8.26
2004	8.4
2005	8.53
2006	8.67
2007	8.81
2008	8.95
2009	9.09
2010	9.23
2011	9.36
2012	9.5
2013	9.64
2014	9.78
2015	9.92