Year	Years
1970	3.3
1971	3.39
1972	3.49
1973	3.59
1974	3.69
1975	3.79
1976	3.89
1977	4
1978	4.11
1979	4.22
1980	4.33
1981	4.44
1982	4.55
1983	4.67
1984	4.79
1985	4.91
1986	5.03
1987	5.15
1988	5.28
1989	5.4
1990	5.53
1991	5.65
1992	5.78
1993	5.91
1994	6.04
1995	6.17
1996	6.31
1997	6.44
1998	6.58
1999	6.72
2000	6.86
2001	7
2002	7.14
2003	7.29
2004	7.44
2005	7.59
2006	7.74
2007	7.89
2008	8.05
2009	8.21
2010	8.36
2011	8.52
2012	8.68
2013	8.84
2014	9.01
2015	9.17