Year	Years
1970	3.36
1971	3.44
1972	3.52
1973	3.6
1974	3.68
1975	3.76
1976	3.84
1977	3.93
1978	4.01
1979	4.1
1980	4.18
1981	4.27
1982	4.36
1983	4.45
1984	4.54
1985	4.63
1986	4.72
1987	4.81
1988	4.91
1989	5
1990	5.09
1991	5.19
1992	5.28
1993	5.38
1994	5.47
1995	5.57
1996	5.67
1997	5.77
1998	5.87
1999	5.98
2000	6.09
2001	6.2
2002	6.31
2003	6.42
2004	6.54
2005	6.66
2006	6.78
2007	6.9
2008	7.02
2009	7.15
2010	7.27
2011	7.4
2012	7.53
2013	7.66
2014	7.79
2015	7.92