Year	Years
1970	3.99
1971	4.08
1972	4.16
1973	4.25
1974	4.34
1975	4.42
1976	4.51
1977	4.6
1978	4.69
1979	4.78
1980	4.87
1981	4.97
1982	5.06
1983	5.15
1984	5.24
1985	5.34
1986	5.43
1987	5.52
1988	5.62
1989	5.71
1990	5.8
1991	5.89
1992	5.99
1993	6.08
1994	6.17
1995	6.26
1996	6.35
1997	6.46
1998	6.56
1999	6.66
2000	6.77
2001	6.88
2002	6.99
2003	7.11
2004	7.22
2005	7.34
2006	7.46
2007	7.58
2008	7.7
2009	7.82
2010	7.95
2011	8.08
2012	8.2
2013	8.33
2014	8.46
2015	8.59