Year	Years
1970	3.96
1971	4.03
1972	4.11
1973	4.18
1974	4.25
1975	4.32
1976	4.39
1977	4.46
1978	4.53
1979	4.59
1980	4.66
1981	4.73
1982	4.79
1983	4.86
1984	4.92
1985	4.98
1986	5.04
1987	5.1
1988	5.16
1989	5.22
1990	5.28
1991	5.34
1992	5.39
1993	5.45
1994	5.5
1995	5.56
1996	5.64
1997	5.73
1998	5.82
1999	5.92
2000	6.01
2001	6.11
2002	6.21
2003	6.31
2004	6.42
2005	6.53
2006	6.64
2007	6.75
2008	6.86
2009	6.97
2010	7.09
2011	7.21
2012	7.33
2013	7.45
2014	7.57
2015	7.7