Year	Years
1970	3.55
1971	3.65
1972	3.74
1973	3.85
1974	3.95
1975	4.05
1976	4.16
1977	4.26
1978	4.37
1979	4.48
1980	4.59
1981	4.7
1982	4.82
1983	4.93
1984	5.05
1985	5.17
1986	5.29
1987	5.41
1988	5.53
1989	5.65
1990	5.77
1991	5.9
1992	6.02
1993	6.15
1994	6.27
1995	6.4
1996	6.54
1997	6.68
1998	6.82
1999	6.97
2000	7.11
2001	7.26
2002	7.41
2003	7.57
2004	7.72
2005	7.87
2006	8.03
2007	8.19
2008	8.35
2009	8.51
2010	8.67
2011	8.83
2012	8.99
2013	9.16
2014	9.32
2015	9.49