Year	Years
1970	3.74
1971	3.82
1972	3.91
1973	4
1974	4.09
1975	4.18
1976	4.28
1977	4.37
1978	4.47
1979	4.57
1980	4.67
1981	4.77
1982	4.87
1983	4.98
1984	5.08
1985	5.19
1986	5.3
1987	5.41
1988	5.52
1989	5.63
1990	5.74
1991	5.86
1992	5.97
1993	6.09
1994	6.21
1995	6.33
1996	6.44
1997	6.55
1998	6.67
1999	6.79
2000	6.91
2001	7.03
2002	7.15
2003	7.27
2004	7.4
2005	7.52
2006	7.65
2007	7.78
2008	7.91
2009	8.04
2010	8.17
2011	8.3
2012	8.43
2013	8.57
2014	8.7
2015	8.84