Year	Years
1970	4.07
1971	4.16
1972	4.26
1973	4.35
1974	4.45
1975	4.55
1976	4.65
1977	4.75
1978	4.85
1979	4.96
1980	5.07
1981	5.17
1982	5.28
1983	5.39
1984	5.51
1985	5.62
1986	5.74
1987	5.85
1988	5.97
1989	6.09
1990	6.21
1991	6.33
1992	6.46
1993	6.58
1994	6.71
1995	6.83
1996	6.96
1997	7.09
1998	7.22
1999	7.35
2000	7.48
2001	7.61
2002	7.74
2003	7.88
2004	8.01
2005	8.15
2006	8.28
2007	8.42
2008	8.56
2009	8.7
2010	8.83
2011	8.97
2012	9.11
2013	9.25
2014	9.39
2015	9.53