Year	Years
1970	3.14
1971	3.22
1972	3.3
1973	3.39
1974	3.48
1975	3.56
1976	3.65
1977	3.75
1978	3.84
1979	3.94
1980	4.04
1981	4.15
1982	4.25
1983	4.36
1984	4.47
1985	4.59
1986	4.71
1987	4.83
1988	4.96
1989	5.09
1990	5.23
1991	5.37
1992	5.51
1993	5.66
1994	5.82
1995	5.98
1996	6.13
1997	6.28
1998	6.43
1999	6.58
2000	6.72
2001	6.87
2002	7.02
2003	7.17
2004	7.31
2005	7.46
2006	7.61
2007	7.75
2008	7.9
2009	8.05
2010	8.19
2011	8.34
2012	8.48
2013	8.63
2014	8.78
2015	8.92