Year	Years
1970	4.66
1971	4.75
1972	4.85
1973	4.94
1974	5.03
1975	5.13
1976	5.22
1977	5.32
1978	5.41
1979	5.51
1980	5.61
1981	5.71
1982	5.8
1983	5.9
1984	6
1985	6.1
1986	6.2
1987	6.3
1988	6.4
1989	6.5
1990	6.59
1991	6.69
1992	6.79
1993	6.89
1994	6.99
1995	7.1
1996	7.22
1997	7.34
1998	7.47
1999	7.59
2000	7.72
2001	7.85
2002	7.98
2003	8.11
2004	8.24
2005	8.37
2006	8.51
2007	8.64
2008	8.78
2009	8.91
2010	9.05
2011	9.19
2012	9.33
2013	9.47
2014	9.6
2015	9.74