Year	Years
1970	3.97
1971	4.06
1972	4.16
1973	4.26
1974	4.36
1975	4.46
1976	4.56
1977	4.67
1978	4.78
1979	4.88
1980	4.99
1981	5.1
1982	5.22
1983	5.33
1984	5.44
1985	5.56
1986	5.68
1987	5.79
1988	5.91
1989	6.03
1990	6.14
1991	6.26
1992	6.38
1993	6.5
1994	6.61
1995	6.73
1996	6.84
1997	6.95
1998	7.06
1999	7.17
2000	7.29
2001	7.4
2002	7.52
2003	7.63
2004	7.75
2005	7.87
2006	7.99
2007	8.11
2008	8.23
2009	8.35
2010	8.47
2011	8.6
2012	8.72
2013	8.85
2014	8.98
2015	9.11