Year	Years
1970	3.31
1971	3.39
1972	3.48
1973	3.57
1974	3.66
1975	3.76
1976	3.85
1977	3.95
1978	4.05
1979	4.15
1980	4.25
1981	4.36
1982	4.46
1983	4.57
1984	4.68
1985	4.79
1986	4.9
1987	5.02
1988	5.14
1989	5.25
1990	5.37
1991	5.49
1992	5.61
1993	5.73
1994	5.85
1995	5.98
1996	6.1
1997	6.22
1998	6.34
1999	6.47
2000	6.59
2001	6.71
2002	6.83
2003	6.96
2004	7.08
2005	7.2
2006	7.32
2007	7.45
2008	7.57
2009	7.69
2010	7.81
2011	7.94
2012	8.06
2013	8.18
2014	8.31
2015	8.43