Year	Years
1970	4.29
1971	4.39
1972	4.5
1973	4.6
1974	4.7
1975	4.81
1976	4.91
1977	5.01
1978	5.11
1979	5.22
1980	5.32
1981	5.42
1982	5.52
1983	5.62
1984	5.72
1985	5.81
1986	5.91
1987	6
1988	6.09
1989	6.18
1990	6.26
1991	6.34
1992	6.42
1993	6.49
1994	6.56
1995	6.63
1996	6.73
1997	6.84
1998	6.95
1999	7.06
2000	7.18
2001	7.3
2002	7.42
2003	7.55
2004	7.68
2005	7.81
2006	7.95
2007	8.08
2008	8.23
2009	8.37
2010	8.51
2011	8.66
2012	8.81
2013	8.96
2014	9.11
2015	9.27