Year	Years
1970	4.04
1971	4.13
1972	4.22
1973	4.32
1974	4.41
1975	4.51
1976	4.61
1977	4.71
1978	4.81
1979	4.91
1980	5.01
1981	5.12
1982	5.23
1983	5.34
1984	5.45
1985	5.56
1986	5.67
1987	5.78
1988	5.9
1989	6.01
1990	6.13
1991	6.25
1992	6.37
1993	6.49
1994	6.61
1995	6.74
1996	6.85
1997	6.97
1998	7.1
1999	7.22
2000	7.34
2001	7.47
2002	7.6
2003	7.73
2004	7.86
2005	7.99
2006	8.12
2007	8.25
2008	8.38
2009	8.51
2010	8.65
2011	8.78
2012	8.92
2013	9.06
2014	9.19
2015	9.33