Year	Years
1970	4.27
1971	4.37
1972	4.46
1973	4.55
1974	4.65
1975	4.74
1976	4.84
1977	4.93
1978	5.03
1979	5.13
1980	5.22
1981	5.32
1982	5.42
1983	5.51
1984	5.61
1985	5.71
1986	5.8
1987	5.9
1988	5.99
1989	6.09
1990	6.18
1991	6.28
1992	6.37
1993	6.46
1994	6.55
1995	6.65
1996	6.74
1997	6.84
1998	6.94
1999	7.04
2000	7.14
2001	7.25
2002	7.36
2003	7.47
2004	7.58
2005	7.7
2006	7.81
2007	7.93
2008	8.06
2009	8.18
2010	8.31
2011	8.44
2012	8.57
2013	8.7
2014	8.83
2015	8.97