Year	Years
1970	3.74
1971	3.83
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	4.97
1984	5.08
1985	5.19
1986	5.29
1987	5.4
1988	5.52
1989	5.63
1990	5.74
1991	5.86
1992	5.98
1993	6.09
1994	6.21
1995	6.34
1996	6.46
1997	6.58
1998	6.71
1999	6.83
2000	6.96
2001	7.09
2002	7.22
2003	7.35
2004	7.48
2005	7.61
2006	7.75
2007	7.88
2008	8.02
2009	8.15
2010	8.29
2011	8.42
2012	8.56
2013	8.7
2014	8.84
2015	8.98