Year	Years
1970	3.57
1971	3.67
1972	3.78
1973	3.89
1974	4
1975	4.12
1976	4.23
1977	4.35
1978	4.47
1979	4.6
1980	4.72
1981	4.85
1982	4.98
1983	5.12
1984	5.26
1985	5.39
1986	5.54
1987	5.68
1988	5.83
1989	5.98
1990	6.13
1991	6.28
1992	6.44
1993	6.59
1994	6.75
1995	6.91
1996	7.07
1997	7.23
1998	7.4
1999	7.56
2000	7.72
2001	7.89
2002	8.05
2003	8.22
2004	8.38
2005	8.55
2006	8.71
2007	8.88
2008	9.05
2009	9.21
2010	9.38
2011	9.55
2012	9.71
2013	9.88
2014	10.05
2015	10.21