Year	Years
1970	4.01
1971	4.1
1972	4.19
1973	4.28
1974	4.38
1975	4.47
1976	4.57
1977	4.67
1978	4.77
1979	4.87
1980	4.97
1981	5.08
1982	5.18
1983	5.29
1984	5.4
1985	5.51
1986	5.62
1987	5.73
1988	5.85
1989	5.96
1990	6.08
1991	6.2
1992	6.32
1993	6.44
1994	6.56
1995	6.68
1996	6.81
1997	6.94
1998	7.07
1999	7.2
2000	7.33
2001	7.46
2002	7.59
2003	7.73
2004	7.86
2005	8
2006	8.14
2007	8.27
2008	8.41
2009	8.55
2010	8.69
2011	8.83
2012	8.97
2013	9.11
2014	9.25
2015	9.4