Year	Years
1970	4.56
1971	4.66
1972	4.76
1973	4.86
1974	4.96
1975	5.07
1976	5.17
1977	5.28
1978	5.39
1979	5.5
1980	5.6
1981	5.71
1982	5.83
1983	5.94
1984	6.05
1985	6.16
1986	6.28
1987	6.39
1988	6.5
1989	6.62
1990	6.73
1991	6.84
1992	6.96
1993	7.07
1994	7.19
1995	7.3
1996	7.41
1997	7.53
1998	7.65
1999	7.76
2000	7.88
2001	8
2002	8.13
2003	8.25
2004	8.37
2005	8.5
2006	8.63
2007	8.76
2008	8.89
2009	9.02
2010	9.15
2011	9.29
2012	9.42
2013	9.56
2014	9.69
2015	9.83