Year	Years
1970	3.31
1971	3.4
1972	3.5
1973	3.6
1974	3.71
1975	3.82
1976	3.93
1977	4.04
1978	4.16
1979	4.28
1980	4.41
1981	4.53
1982	4.67
1983	4.8
1984	4.94
1985	5.08
1986	5.23
1987	5.38
1988	5.53
1989	5.69
1990	5.85
1991	6.01
1992	6.17
1993	6.34
1994	6.51
1995	6.68
1996	6.82
1997	6.96
1998	7.1
1999	7.23
2000	7.37
2001	7.5
2002	7.64
2003	7.77
2004	7.91
2005	8.04
2006	8.17
2007	8.3
2008	8.43
2009	8.57
2010	8.7
2011	8.83
2012	8.96
2013	9.09
2014	9.22
2015	9.35