Year	Years
1970	3.28
1971	3.38
1972	3.47
1973	3.57
1974	3.67
1975	3.77
1976	3.88
1977	3.98
1978	4.09
1979	4.2
1980	4.32
1981	4.43
1982	4.55
1983	4.67
1984	4.79
1985	4.91
1986	5.04
1987	5.17
1988	5.3
1989	5.43
1990	5.56
1991	5.7
1992	5.84
1993	5.98
1994	6.12
1995	6.26
1996	6.41
1997	6.56
1998	6.7
1999	6.86
2000	7.01
2001	7.16
2002	7.32
2003	7.48
2004	7.63
2005	7.79
2006	7.96
2007	8.12
2008	8.28
2009	8.45
2010	8.61
2011	8.78
2012	8.94
2013	9.11
2014	9.28
2015	9.45