Year	Years
1970	4.43
1971	4.53
1972	4.63
1973	4.73
1974	4.83
1975	4.93
1976	5.03
1977	5.14
1978	5.25
1979	5.36
1980	5.47
1981	5.58
1982	5.69
1983	5.81
1984	5.92
1985	6.04
1986	6.16
1987	6.27
1988	6.39
1989	6.52
1990	6.64
1991	6.76
1992	6.89
1993	7.01
1994	7.14
1995	7.27
1996	7.39
1997	7.52
1998	7.65
1999	7.77
2000	7.9
2001	8.04
2002	8.17
2003	8.3
2004	8.43
2005	8.57
2006	8.7
2007	8.84
2008	8.97
2009	9.11
2010	9.25
2011	9.39
2012	9.52
2013	9.66
2014	9.8
2015	9.94