Year	Years
1970	4.24
1971	4.34
1972	4.44
1973	4.55
1974	4.66
1975	4.76
1976	4.87
1977	4.99
1978	5.1
1979	5.22
1980	5.33
1981	5.45
1982	5.57
1983	5.7
1984	5.82
1985	5.95
1986	6.08
1987	6.21
1988	6.34
1989	6.47
1990	6.6
1991	6.74
1992	6.87
1993	7.01
1994	7.14
1995	7.28
1996	7.41
1997	7.54
1998	7.68
1999	7.81
2000	7.95
2001	8.08
2002	8.22
2003	8.35
2004	8.49
2005	8.63
2006	8.77
2007	8.9
2008	9.04
2009	9.18
2010	9.32
2011	9.45
2012	9.59
2013	9.73
2014	9.87
2015	10.01