Year	Years
1970	3.22
1971	3.31
1972	3.41
1973	3.5
1974	3.6
1975	3.71
1976	3.81
1977	3.92
1978	4.03
1979	4.14
1980	4.25
1981	4.36
1982	4.48
1983	4.6
1984	4.72
1985	4.84
1986	4.97
1987	5.09
1988	5.22
1989	5.35
1990	5.48
1991	5.62
1992	5.75
1993	5.89
1994	6.02
1995	6.16
1996	6.3
1997	6.44
1998	6.58
1999	6.73
2000	6.87
2001	7.02
2002	7.17
2003	7.32
2004	7.47
2005	7.62
2006	7.77
2007	7.93
2008	8.08
2009	8.24
2010	8.4
2011	8.56
2012	8.72
2013	8.88
2014	9.04
2015	9.2