Year	Years
1970	3.82
1971	3.91
1972	4
1973	4.1
1974	4.19
1975	4.29
1976	4.39
1977	4.49
1978	4.59
1979	4.7
1980	4.81
1981	4.91
1982	5.02
1983	5.14
1984	5.25
1985	5.37
1986	5.48
1987	5.6
1988	5.72
1989	5.84
1990	5.97
1991	6.09
1992	6.22
1993	6.34
1994	6.47
1995	6.6
1996	6.73
1997	6.85
1998	6.98
1999	7.1
2000	7.23
2001	7.36
2002	7.49
2003	7.62
2004	7.75
2005	7.88
2006	8.01
2007	8.14
2008	8.28
2009	8.41
2010	8.55
2011	8.68
2012	8.82
2013	8.96
2014	9.09
2015	9.23