Year	Years
1970	3.31
1971	3.4
1972	3.5
1973	3.6
1974	3.7
1975	3.8
1976	3.91
1977	4.01
1978	4.12
1979	4.24
1980	4.35
1981	4.47
1982	4.58
1983	4.7
1984	4.83
1985	4.95
1986	5.08
1987	5.21
1988	5.34
1989	5.47
1990	5.61
1991	5.75
1992	5.89
1993	6.03
1994	6.17
1995	6.32
1996	6.47
1997	6.61
1998	6.77
1999	6.92
2000	7.07
2001	7.23
2002	7.38
2003	7.54
2004	7.7
2005	7.86
2006	8.02
2007	8.18
2008	8.35
2009	8.51
2010	8.68
2011	8.84
2012	9.01
2013	9.18
2014	9.34
2015	9.51