Year	Years
1970	5.18
1971	5.26
1972	5.35
1973	5.44
1974	5.52
1975	5.61
1976	5.69
1977	5.77
1978	5.85
1979	5.92
1980	6
1981	6.07
1982	6.14
1983	6.2
1984	6.27
1985	6.32
1986	6.37
1987	6.42
1988	6.46
1989	6.49
1990	6.52
1991	6.54
1992	6.55
1993	6.55
1994	6.55
1995	6.54
1996	6.61
1997	6.69
1998	6.78
1999	6.87
2000	6.97
2001	7.07
2002	7.17
2003	7.27
2004	7.38
2005	7.49
2006	7.61
2007	7.72
2008	7.84
2009	7.96
2010	8.08
2011	8.21
2012	8.33
2013	8.46
2014	8.59
2015	8.72