Year	Years
1970	4.39
1971	4.48
1972	4.58
1973	4.67
1974	4.77
1975	4.86
1976	4.95
1977	5.05
1978	5.14
1979	5.24
1980	5.33
1981	5.42
1982	5.51
1983	5.6
1984	5.69
1985	5.78
1986	5.86
1987	5.95
1988	6.03
1989	6.11
1990	6.18
1991	6.26
1992	6.33
1993	6.4
1994	6.48
1995	6.55
1996	6.68
1997	6.81
1998	6.94
1999	7.08
2000	7.22
2001	7.36
2002	7.51
2003	7.65
2004	7.8
2005	7.95
2006	8.11
2007	8.26
2008	8.42
2009	8.57
2010	8.73
2011	8.89
2012	9.05
2013	9.21
2014	9.37
2015	9.53