Year	Years
1970	3.39
1971	3.47
1972	3.55
1973	3.63
1974	3.71
1975	3.8
1976	3.88
1977	3.97
1978	4.06
1979	4.15
1980	4.24
1981	4.33
1982	4.42
1983	4.52
1984	4.62
1985	4.71
1986	4.81
1987	4.91
1988	5.02
1989	5.12
1990	5.23
1991	5.33
1992	5.44
1993	5.55
1994	5.66
1995	5.78
1996	5.89
1997	6.01
1998	6.12
1999	6.24
2000	6.36
2001	6.48
2002	6.61
2003	6.73
2004	6.86
2005	6.99
2006	7.12
2007	7.25
2008	7.38
2009	7.51
2010	7.64
2011	7.78
2012	7.92
2013	8.05
2014	8.19
2015	8.33