Year	Years
1970	4.35
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	5.71
1984	5.83
1985	5.94
1986	6.06
1987	6.18
1988	6.3
1989	6.42
1990	6.55
1991	6.67
1992	6.8
1993	6.92
1994	7.05
1995	7.18
1996	7.31
1997	7.44
1998	7.57
1999	7.7
2000	7.84
2001	7.97
2002	8.11
2003	8.24
2004	8.38
2005	8.51
2006	8.65
2007	8.79
2008	8.93
2009	9.06
2010	9.2
2011	9.34
2012	9.48
2013	9.62
2014	9.76
2015	9.9