Year	Years
1970	4.09
1971	4.19
1972	4.28
1973	4.38
1974	4.48
1975	4.58
1976	4.68
1977	4.78
1978	4.88
1979	4.99
1980	5.1
1981	5.2
1982	5.32
1983	5.43
1984	5.54
1985	5.65
1986	5.77
1987	5.89
1988	6.01
1989	6.13
1990	6.25
1991	6.37
1992	6.5
1993	6.62
1994	6.75
1995	6.88
1996	7.01
1997	7.14
1998	7.27
1999	7.4
2000	7.53
2001	7.67
2002	7.8
2003	7.94
2004	8.07
2005	8.21
2006	8.34
2007	8.48
2008	8.62
2009	8.76
2010	8.89
2011	9.03
2012	9.17
2013	9.31
2014	9.45
2015	9.59