Year	Years
1970	3.72
1971	3.81
1972	3.9
1973	3.99
1974	4.08
1975	4.18
1976	4.27
1977	4.37
1978	4.47
1979	4.58
1980	4.68
1981	4.79
1982	4.9
1983	5.01
1984	5.12
1985	5.24
1986	5.36
1987	5.48
1988	5.6
1989	5.73
1990	5.86
1991	5.99
1992	6.12
1993	6.26
1994	6.4
1995	6.55
1996	6.69
1997	6.83
1998	6.97
1999	7.11
2000	7.25
2001	7.39
2002	7.53
2003	7.68
2004	7.82
2005	7.96
2006	8.1
2007	8.25
2008	8.39
2009	8.53
2010	8.68
2011	8.82
2012	8.96
2013	9.11
2014	9.25
2015	9.4