Year	Years
1970	3.79
1971	3.87
1972	3.95
1973	4.03
1974	4.11
1975	4.19
1976	4.28
1977	4.36
1978	4.44
1979	4.52
1980	4.6
1981	4.69
1982	4.77
1983	4.85
1984	4.93
1985	5
1986	5.08
1987	5.16
1988	5.23
1989	5.3
1990	5.38
1991	5.44
1992	5.51
1993	5.58
1994	5.64
1995	5.7
1996	5.77
1997	5.84
1998	5.91
1999	5.98
2000	6.06
2001	6.15
2002	6.23
2003	6.32
2004	6.41
2005	6.5
2006	6.6
2007	6.69
2008	6.79
2009	6.9
2010	7
2011	7.11
2012	7.22
2013	7.33
2014	7.44
2015	7.56