Year	Years
1970	3.79
1971	3.88
1972	3.96
1973	4.05
1974	4.14
1975	4.22
1976	4.31
1977	4.4
1978	4.5
1979	4.59
1980	4.68
1981	4.77
1982	4.87
1983	4.96
1984	5.06
1985	5.15
1986	5.25
1987	5.35
1988	5.44
1989	5.54
1990	5.64
1991	5.74
1992	5.84
1993	5.93
1994	6.03
1995	6.13
1996	6.24
1997	6.36
1998	6.47
1999	6.59
2000	6.7
2001	6.82
2002	6.94
2003	7.07
2004	7.19
2005	7.31
2006	7.44
2007	7.56
2008	7.69
2009	7.82
2010	7.95
2011	8.08
2012	8.21
2013	8.34
2014	8.47
2015	8.61