Year	Years
1970	3.55
1971	3.65
1972	3.75
1973	3.86
1974	3.96
1975	4.07
1976	4.18
1977	4.3
1978	4.41
1979	4.53
1980	4.65
1981	4.78
1982	4.9
1983	5.03
1984	5.16
1985	5.29
1986	5.42
1987	5.56
1988	5.7
1989	5.84
1990	5.98
1991	6.12
1992	6.27
1993	6.41
1994	6.56
1995	6.71
1996	6.87
1997	7.02
1998	7.17
1999	7.33
2000	7.49
2001	7.65
2002	7.81
2003	7.97
2004	8.13
2005	8.3
2006	8.46
2007	8.63
2008	8.79
2009	8.96
2010	9.13
2011	9.3
2012	9.46
2013	9.63
2014	9.8
2015	9.97