Year	Years
1970	3.92
1971	4.01
1972	4.1
1973	4.2
1974	4.3
1975	4.39
1976	4.49
1977	4.59
1978	4.7
1979	4.8
1980	4.91
1981	5.02
1982	5.13
1983	5.24
1984	5.36
1985	5.47
1986	5.59
1987	5.71
1988	5.83
1989	5.95
1990	6.07
1991	6.2
1992	6.33
1993	6.45
1994	6.58
1995	6.72
1996	6.85
1997	6.98
1998	7.12
1999	7.25
2000	7.39
2001	7.53
2002	7.66
2003	7.8
2004	7.94
2005	8.08
2006	8.22
2007	8.36
2008	8.51
2009	8.65
2010	8.79
2011	8.93
2012	9.08
2013	9.22
2014	9.36
2015	9.51