Year	Years
1970	3.49
1971	3.57
1972	3.66
1973	3.74
1974	3.83
1975	3.91
1976	4
1977	4.09
1978	4.18
1979	4.27
1980	4.37
1981	4.46
1982	4.56
1983	4.66
1984	4.76
1985	4.86
1986	4.97
1987	5.07
1988	5.18
1989	5.29
1990	5.4
1991	5.51
1992	5.62
1993	5.74
1994	5.85
1995	5.97
1996	6.1
1997	6.23
1998	6.35
1999	6.48
2000	6.61
2001	6.74
2002	6.87
2003	7.01
2004	7.14
2005	7.27
2006	7.41
2007	7.55
2008	7.68
2009	7.82
2010	7.96
2011	8.1
2012	8.24
2013	8.38
2014	8.52
2015	8.66