Year	Years
1970	3.78
1971	3.87
1972	3.95
1973	4.03
1974	4.11
1975	4.19
1976	4.28
1977	4.36
1978	4.45
1979	4.53
1980	4.62
1981	4.71
1982	4.79
1983	4.88
1984	4.97
1985	5.06
1986	5.15
1987	5.24
1988	5.33
1989	5.42
1990	5.51
1991	5.6
1992	5.69
1993	5.78
1994	5.87
1995	5.96
1996	6.06
1997	6.16
1998	6.26
1999	6.37
2000	6.47
2001	6.58
2002	6.69
2003	6.8
2004	6.92
2005	7.03
2006	7.15
2007	7.27
2008	7.39
2009	7.52
2010	7.64
2011	7.77
2012	7.9
2013	8.03
2014	8.16
2015	8.29