Year	Years
1970	4.38
1971	4.48
1972	4.59
1973	4.7
1974	4.81
1975	4.92
1976	5.03
1977	5.15
1978	5.26
1979	5.38
1980	5.5
1981	5.62
1982	5.75
1983	5.87
1984	6
1985	6.13
1986	6.25
1987	6.38
1988	6.51
1989	6.64
1990	6.78
1991	6.91
1992	7.04
1993	7.17
1994	7.3
1995	7.44
1996	7.56
1997	7.68
1998	7.81
1999	7.93
2000	8.06
2001	8.18
2002	8.31
2003	8.44
2004	8.56
2005	8.69
2006	8.82
2007	8.95
2008	9.08
2009	9.21
2010	9.35
2011	9.48
2012	9.61
2013	9.75
2014	9.88
2015	10.02