Year	Years
1970	3.91
1971	3.99
1972	4.08
1973	4.17
1974	4.25
1975	4.34
1976	4.43
1977	4.52
1978	4.61
1979	4.7
1980	4.79
1981	4.88
1982	4.97
1983	5.06
1984	5.15
1985	5.23
1986	5.32
1987	5.41
1988	5.49
1989	5.58
1990	5.66
1991	5.74
1992	5.82
1993	5.89
1994	5.97
1995	6.04
1996	6.1
1997	6.16
1998	6.23
1999	6.3
2000	6.37
2001	6.44
2002	6.52
2003	6.6
2004	6.69
2005	6.78
2006	6.86
2007	6.96
2008	7.05
2009	7.15
2010	7.25
2011	7.35
2012	7.46
2013	7.57
2014	7.68
2015	7.79