Year	Years
1970	3.4
1971	3.48
1972	3.57
1973	3.65
1974	3.74
1975	3.82
1976	3.91
1977	4
1978	4.09
1979	4.18
1980	4.28
1981	4.37
1982	4.47
1983	4.57
1984	4.66
1985	4.76
1986	4.86
1987	4.96
1988	5.06
1989	5.16
1990	5.26
1991	5.36
1992	5.47
1993	5.57
1994	5.67
1995	5.77
1996	5.87
1997	5.97
1998	6.06
1999	6.16
2000	6.27
2001	6.37
2002	6.47
2003	6.58
2004	6.68
2005	6.79
2006	6.9
2007	7.01
2008	7.13
2009	7.24
2010	7.36
2011	7.48
2012	7.6
2013	7.72
2014	7.84
2015	7.96