Year	Years
1970	4.08
1971	4.18
1972	4.27
1973	4.37
1974	4.47
1975	4.58
1976	4.68
1977	4.79
1978	4.9
1979	5.01
1980	5.12
1981	5.23
1982	5.35
1983	5.46
1984	5.58
1985	5.71
1986	5.83
1987	5.96
1988	6.09
1989	6.22
1990	6.35
1991	6.49
1992	6.63
1993	6.78
1994	6.93
1995	7.08
1996	7.22
1997	7.35
1998	7.48
1999	7.61
2000	7.75
2001	7.88
2002	8.02
2003	8.15
2004	8.29
2005	8.43
2006	8.56
2007	8.7
2008	8.84
2009	8.98
2010	9.12
2011	9.26
2012	9.4
2013	9.54
2014	9.68
2015	9.82