Year	Years
1970	4.35
1971	4.44
1972	4.54
1973	4.63
1974	4.73
1975	4.83
1976	4.92
1977	5.02
1978	5.12
1979	5.22
1980	5.32
1981	5.42
1982	5.52
1983	5.62
1984	5.73
1985	5.83
1986	5.93
1987	6.03
1988	6.13
1989	6.23
1990	6.33
1991	6.43
1992	6.53
1993	6.62
1994	6.72
1995	6.82
1996	6.91
1997	7.01
1998	7.11
1999	7.22
2000	7.33
2001	7.43
2002	7.54
2003	7.66
2004	7.77
2005	7.89
2006	8
2007	8.12
2008	8.24
2009	8.36
2010	8.49
2011	8.61
2012	8.74
2013	8.86
2014	8.99
2015	9.12