Year	Years
1970	3.26
1971	3.36
1972	3.46
1973	3.56
1974	3.66
1975	3.77
1976	3.88
1977	3.99
1978	4.1
1979	4.22
1980	4.33
1981	4.45
1982	4.58
1983	4.7
1984	4.83
1985	4.96
1986	5.09
1987	5.23
1988	5.37
1989	5.51
1990	5.66
1991	5.81
1992	5.97
1993	6.13
1994	6.29
1995	6.46
1996	6.61
1997	6.76
1998	6.91
1999	7.06
2000	7.22
2001	7.37
2002	7.53
2003	7.69
2004	7.85
2005	8.01
2006	8.17
2007	8.34
2008	8.5
2009	8.67
2010	8.83
2011	9
2012	9.16
2013	9.33
2014	9.5
2015	9.67