Year	Years
1970	3.73
1971	3.79
1972	3.85
1973	3.91
1974	3.97
1975	4.02
1976	4.08
1977	4.13
1978	4.18
1979	4.22
1980	4.27
1981	4.31
1982	4.35
1983	4.39
1984	4.42
1985	4.45
1986	4.48
1987	4.5
1988	4.52
1989	4.53
1990	4.54
1991	4.54
1992	4.54
1993	4.54
1994	4.54
1995	4.53
1996	4.61
1997	4.7
1998	4.78
1999	4.87
2000	4.97
2001	5.07
2002	5.17
2003	5.27
2004	5.38
2005	5.49
2006	5.61
2007	5.72
2008	5.84
2009	5.97
2010	6.09
2011	6.22
2012	6.35
2013	6.48
2014	6.62
2015	6.75