Year	Years
1970	3.18
1971	3.25
1972	3.33
1973	3.41
1974	3.49
1975	3.57
1976	3.65
1977	3.74
1978	3.82
1979	3.91
1980	4
1981	4.08
1982	4.17
1983	4.27
1984	4.36
1985	4.45
1986	4.54
1987	4.64
1988	4.74
1989	4.83
1990	4.93
1991	5.03
1992	5.13
1993	5.23
1994	5.33
1995	5.43
1996	5.53
1997	5.63
1998	5.74
1999	5.84
2000	5.95
2001	6.06
2002	6.17
2003	6.28
2004	6.4
2005	6.51
2006	6.63
2007	6.75
2008	6.87
2009	6.99
2010	7.11
2011	7.24
2012	7.36
2013	7.49
2014	7.62
2015	7.75