Year	Years
1970	3.68
1971	3.78
1972	3.89
1973	4
1974	4.11
1975	4.22
1976	4.33
1977	4.45
1978	4.57
1979	4.69
1980	4.81
1981	4.94
1982	5.07
1983	5.2
1984	5.33
1985	5.47
1986	5.6
1987	5.74
1988	5.88
1989	6.03
1990	6.17
1991	6.32
1992	6.47
1993	6.62
1994	6.78
1995	6.94
1996	7.1
1997	7.27
1998	7.43
1999	7.6
2000	7.77
2001	7.94
2002	8.11
2003	8.29
2004	8.46
2005	8.63
2006	8.81
2007	8.98
2008	9.16
2009	9.33
2010	9.51
2011	9.68
2012	9.85
2013	10.03
2014	10.2
2015	10.37