Year	Years
1970	3.73
1971	3.81
1972	3.91
1973	4
1974	4.09
1975	4.19
1976	4.28
1977	4.38
1978	4.48
1979	4.58
1980	4.69
1981	4.79
1982	4.9
1983	5.01
1984	5.12
1985	5.24
1986	5.35
1987	5.47
1988	5.58
1989	5.7
1990	5.82
1991	5.95
1992	6.07
1993	6.2
1994	6.33
1995	6.46
1996	6.58
1997	6.71
1998	6.84
1999	6.96
2000	7.09
2001	7.22
2002	7.35
2003	7.49
2004	7.62
2005	7.75
2006	7.88
2007	8.02
2008	8.15
2009	8.29
2010	8.43
2011	8.56
2012	8.7
2013	8.84
2014	8.98
2015	9.11