Year	Years
1970	3.97
1971	4.08
1972	4.19
1973	4.3
1974	4.41
1975	4.53
1976	4.64
1977	4.76
1978	4.88
1979	5.01
1980	5.13
1981	5.26
1982	5.38
1983	5.51
1984	5.64
1985	5.77
1986	5.91
1987	6.04
1988	6.17
1989	6.31
1990	6.44
1991	6.58
1992	6.72
1993	6.85
1994	6.99
1995	7.12
1996	7.26
1997	7.4
1998	7.54
1999	7.68
2000	7.83
2001	7.97
2002	8.12
2003	8.27
2004	8.42
2005	8.58
2006	8.73
2007	8.89
2008	9.04
2009	9.2
2010	9.36
2011	9.52
2012	9.68
2013	9.84
2014	10
2015	10.16