Year	Years
1970	4.12
1971	4.21
1972	4.3
1973	4.39
1974	4.49
1975	4.58
1976	4.68
1977	4.78
1978	4.88
1979	4.98
1980	5.08
1981	5.18
1982	5.28
1983	5.39
1984	5.49
1985	5.6
1986	5.71
1987	5.82
1988	5.93
1989	6.04
1990	6.15
1991	6.26
1992	6.37
1993	6.49
1994	6.6
1995	6.71
1996	6.83
1997	6.95
1998	7.07
1999	7.19
2000	7.31
2001	7.43
2002	7.56
2003	7.68
2004	7.81
2005	7.94
2006	8.07
2007	8.2
2008	8.33
2009	8.46
2010	8.6
2011	8.73
2012	8.87
2013	9.01
2014	9.15
2015	9.29