Year	Years
1970	3.02
1971	3.11
1972	3.2
1973	3.3
1974	3.4
1975	3.5
1976	3.6
1977	3.71
1978	3.82
1979	3.93
1980	4.04
1981	4.16
1982	4.28
1983	4.41
1984	4.53
1985	4.66
1986	4.8
1987	4.94
1988	5.08
1989	5.22
1990	5.37
1991	5.52
1992	5.68
1993	5.84
1994	6
1995	6.17
1996	6.36
1997	6.54
1998	6.73
1999	6.91
2000	7.09
2001	7.28
2002	7.46
2003	7.65
2004	7.84
2005	8.02
2006	8.21
2007	8.4
2008	8.58
2009	8.77
2010	8.95
2011	9.14
2012	9.32
2013	9.51
2014	9.69
2015	9.87