Year	Years
1970	3.24
1971	3.32
1972	3.4
1973	3.48
1974	3.56
1975	3.64
1976	3.72
1977	3.81
1978	3.9
1979	3.98
1980	4.07
1981	4.16
1982	4.26
1983	4.35
1984	4.45
1985	4.54
1986	4.64
1987	4.74
1988	4.84
1989	4.94
1990	5.05
1991	5.15
1992	5.26
1993	5.37
1994	5.48
1995	5.59
1996	5.7
1997	5.82
1998	5.94
1999	6.06
2000	6.18
2001	6.3
2002	6.42
2003	6.55
2004	6.67
2005	6.8
2006	6.93
2007	7.05
2008	7.18
2009	7.32
2010	7.45
2011	7.58
2012	7.71
2013	7.85
2014	7.98
2015	8.12