Year	Years
1970	4.36
1971	4.45
1972	4.55
1973	4.65
1974	4.76
1975	4.86
1976	4.97
1977	5.07
1978	5.18
1979	5.29
1980	5.4
1981	5.51
1982	5.63
1983	5.75
1984	5.86
1985	5.98
1986	6.1
1987	6.23
1988	6.35
1989	6.47
1990	6.6
1991	6.73
1992	6.86
1993	6.99
1994	7.12
1995	7.25
1996	7.38
1997	7.5
1998	7.63
1999	7.76
2000	7.89
2001	8.02
2002	8.15
2003	8.29
2004	8.42
2005	8.55
2006	8.68
2007	8.82
2008	8.95
2009	9.09
2010	9.22
2011	9.36
2012	9.49
2013	9.63
2014	9.77
2015	9.91