Year	Years
1970	4.21
1971	4.31
1972	4.4
1973	4.49
1974	4.59
1975	4.68
1976	4.78
1977	4.88
1978	4.98
1979	5.08
1980	5.18
1981	5.28
1982	5.38
1983	5.48
1984	5.59
1985	5.69
1986	5.8
1987	5.9
1988	6
1989	6.11
1990	6.21
1991	6.32
1992	6.42
1993	6.52
1994	6.63
1995	6.73
1996	6.83
1997	6.94
1998	7.05
1999	7.16
2000	7.27
2001	7.39
2002	7.5
2003	7.62
2004	7.74
2005	7.86
2006	7.99
2007	8.11
2008	8.24
2009	8.36
2010	8.49
2011	8.62
2012	8.76
2013	8.89
2014	9.02
2015	9.16