Year	Years
1970	3.07
1971	3.16
1972	3.25
1973	3.34
1974	3.43
1975	3.53
1976	3.63
1977	3.74
1978	3.84
1979	3.95
1980	4.06
1981	4.18
1982	4.3
1983	4.42
1984	4.55
1985	4.68
1986	4.81
1987	4.95
1988	5.08
1989	5.23
1990	5.37
1991	5.51
1992	5.66
1993	5.81
1994	5.95
1995	6.1
1996	6.22
1997	6.34
1998	6.46
1999	6.58
2000	6.71
2001	6.83
2002	6.96
2003	7.08
2004	7.21
2005	7.34
2006	7.47
2007	7.6
2008	7.73
2009	7.87
2010	8
2011	8.13
2012	8.27
2013	8.4
2014	8.54
2015	8.68