Year	Years
1970	3.64
1971	3.74
1972	3.84
1973	3.94
1974	4.05
1975	4.16
1976	4.27
1977	4.39
1978	4.5
1979	4.62
1980	4.74
1981	4.86
1982	4.98
1983	5.11
1984	5.24
1985	5.36
1986	5.5
1987	5.63
1988	5.76
1989	5.9
1990	6.04
1991	6.18
1992	6.32
1993	6.47
1994	6.61
1995	6.76
1996	6.91
1997	7.07
1998	7.22
1999	7.38
2000	7.53
2001	7.69
2002	7.85
2003	8.01
2004	8.18
2005	8.34
2006	8.5
2007	8.67
2008	8.84
2009	9
2010	9.17
2011	9.34
2012	9.5
2013	9.67
2014	9.84
2015	10.01