Year	Years
1970	3.46
1971	3.56
1972	3.66
1973	3.77
1974	3.87
1975	3.98
1976	4.09
1977	4.2
1978	4.31
1979	4.43
1980	4.55
1981	4.67
1982	4.79
1983	4.92
1984	5.04
1985	5.17
1986	5.3
1987	5.44
1988	5.57
1989	5.71
1990	5.85
1991	5.99
1992	6.13
1993	6.28
1994	6.42
1995	6.57
1996	6.72
1997	6.87
1998	7.02
1999	7.18
2000	7.33
2001	7.49
2002	7.65
2003	7.81
2004	7.97
2005	8.13
2006	8.29
2007	8.46
2008	8.62
2009	8.79
2010	8.95
2011	9.12
2012	9.29
2013	9.45
2014	9.62
2015	9.79