Year	Years
1970	3.7
1971	3.79
1972	3.89
1973	3.98
1974	4.08
1975	4.18
1976	4.28
1977	4.38
1978	4.49
1979	4.59
1980	4.7
1981	4.81
1982	4.91
1983	5.03
1984	5.14
1985	5.25
1986	5.36
1987	5.48
1988	5.59
1989	5.71
1990	5.82
1991	5.94
1992	6.05
1993	6.17
1994	6.28
1995	6.39
1996	6.5
1997	6.61
1998	6.72
1999	6.83
2000	6.94
2001	7.05
2002	7.16
2003	7.28
2004	7.39
2005	7.5
2006	7.62
2007	7.73
2008	7.85
2009	7.96
2010	8.08
2011	8.2
2012	8.32
2013	8.44
2014	8.56
2015	8.68