Year	Years
1970	3.38
1971	3.46
1972	3.55
1973	3.63
1974	3.72
1975	3.8
1976	3.89
1977	3.98
1978	4.08
1979	4.17
1980	4.27
1981	4.36
1982	4.46
1983	4.56
1984	4.67
1985	4.77
1986	4.88
1987	4.99
1988	5.09
1989	5.21
1990	5.32
1991	5.43
1992	5.55
1993	5.67
1994	5.78
1995	5.9
1996	6.02
1997	6.14
1998	6.26
1999	6.38
2000	6.5
2001	6.63
2002	6.75
2003	6.88
2004	7
2005	7.13
2006	7.26
2007	7.39
2008	7.52
2009	7.65
2010	7.78
2011	7.92
2012	8.05
2013	8.19
2014	8.32
2015	8.46