Year	Years
1970	3.41
1971	3.49
1972	3.57
1973	3.65
1974	3.74
1975	3.82
1976	3.91
1977	4
1978	4.09
1979	4.18
1980	4.28
1981	4.37
1982	4.47
1983	4.57
1984	4.67
1985	4.77
1986	4.87
1987	4.98
1988	5.08
1989	5.19
1990	5.3
1991	5.41
1992	5.52
1993	5.63
1994	5.75
1995	5.86
1996	5.98
1997	6.1
1998	6.22
1999	6.34
2000	6.46
2001	6.58
2002	6.71
2003	6.83
2004	6.96
2005	7.08
2006	7.21
2007	7.34
2008	7.47
2009	7.6
2010	7.73
2011	7.87
2012	8
2013	8.13
2014	8.27
2015	8.4