Year	Years
1970	3.33
1971	3.43
1972	3.53
1973	3.63
1974	3.73
1975	3.84
1976	3.95
1977	4.06
1978	4.17
1979	4.29
1980	4.41
1981	4.53
1982	4.65
1983	4.78
1984	4.91
1985	5.04
1986	5.17
1987	5.31
1988	5.44
1989	5.58
1990	5.72
1991	5.87
1992	6.01
1993	6.16
1994	6.31
1995	6.46
1996	6.6
1997	6.75
1998	6.9
1999	7.05
2000	7.2
2001	7.36
2002	7.51
2003	7.67
2004	7.82
2005	7.98
2006	8.14
2007	8.3
2008	8.46
2009	8.62
2010	8.78
2011	8.95
2012	9.11
2013	9.27
2014	9.44
2015	9.6