Year	Years
1970	3.59
1971	3.69
1972	3.79
1973	3.89
1974	4
1975	4.11
1976	4.22
1977	4.33
1978	4.45
1979	4.57
1980	4.69
1981	4.81
1982	4.93
1983	5.06
1984	5.18
1985	5.31
1986	5.45
1987	5.58
1988	5.72
1989	5.85
1990	5.99
1991	6.14
1992	6.28
1993	6.43
1994	6.57
1995	6.72
1996	6.87
1997	7.03
1998	7.18
1999	7.34
2000	7.49
2001	7.65
2002	7.81
2003	7.97
2004	8.13
2005	8.3
2006	8.46
2007	8.63
2008	8.79
2009	8.96
2010	9.12
2011	9.29
2012	9.46
2013	9.63
2014	9.79
2015	9.96