Year	Years
1970	4.17
1971	4.27
1972	4.38
1973	4.48
1974	4.58
1975	4.69
1976	4.8
1977	4.9
1978	5.01
1979	5.12
1980	5.22
1981	5.33
1982	5.44
1983	5.54
1984	5.65
1985	5.76
1986	5.86
1987	5.96
1988	6.06
1989	6.16
1990	6.26
1991	6.35
1992	6.44
1993	6.53
1994	6.61
1995	6.69
1996	6.8
1997	6.91
1998	7.03
1999	7.15
2000	7.28
2001	7.41
2002	7.54
2003	7.68
2004	7.81
2005	7.95
2006	8.1
2007	8.24
2008	8.39
2009	8.54
2010	8.69
2011	8.85
2012	9
2013	9.16
2014	9.32
2015	9.48