Year	Years
1970	3.76
1971	3.84
1972	3.93
1973	4.02
1974	4.11
1975	4.2
1976	4.29
1977	4.38
1978	4.47
1979	4.57
1980	4.66
1981	4.75
1982	4.85
1983	4.95
1984	5.04
1985	5.14
1986	5.24
1987	5.35
1988	5.45
1989	5.56
1990	5.68
1991	5.8
1992	5.92
1993	6.05
1994	6.19
1995	6.33
1996	6.49
1997	6.65
1998	6.81
1999	6.97
2000	7.13
2001	7.3
2002	7.46
2003	7.63
2004	7.79
2005	7.96
2006	8.13
2007	8.3
2008	8.47
2009	8.63
2010	8.8
2011	8.97
2012	9.14
2013	9.31
2014	9.48
2015	9.65