Year	Years
1970	3.45
1971	3.54
1972	3.64
1973	3.74
1974	3.84
1975	3.94
1976	4.05
1977	4.15
1978	4.27
1979	4.38
1980	4.5
1981	4.62
1982	4.74
1983	4.87
1984	5
1985	5.13
1986	5.26
1987	5.4
1988	5.54
1989	5.69
1990	5.83
1991	5.98
1992	6.13
1993	6.27
1994	6.42
1995	6.57
1996	6.7
1997	6.84
1998	6.97
1999	7.11
2000	7.24
2001	7.38
2002	7.51
2003	7.65
2004	7.79
2005	7.92
2006	8.06
2007	8.2
2008	8.34
2009	8.47
2010	8.61
2011	8.75
2012	8.89
2013	9.03
2014	9.16
2015	9.3