Year	Years
1970	3.28
1971	3.37
1972	3.46
1973	3.55
1974	3.65
1975	3.74
1976	3.84
1977	3.94
1978	4.04
1979	4.14
1980	4.24
1981	4.34
1982	4.45
1983	4.56
1984	4.66
1985	4.77
1986	4.88
1987	4.99
1988	5.1
1989	5.21
1990	5.32
1991	5.44
1992	5.55
1993	5.67
1994	5.78
1995	5.9
1996	6.03
1997	6.16
1998	6.3
1999	6.44
2000	6.58
2001	6.72
2002	6.86
2003	7.01
2004	7.16
2005	7.31
2006	7.46
2007	7.61
2008	7.76
2009	7.92
2010	8.08
2011	8.24
2012	8.4
2013	8.56
2014	8.72
2015	8.88