Year	Years
1970	3.37
1971	3.47
1972	3.57
1973	3.67
1974	3.78
1975	3.89
1976	4
1977	4.12
1978	4.23
1979	4.35
1980	4.48
1981	4.6
1982	4.73
1983	4.86
1984	4.99
1985	5.12
1986	5.26
1987	5.4
1988	5.54
1989	5.69
1990	5.83
1991	5.98
1992	6.13
1993	6.28
1994	6.43
1995	6.58
1996	6.73
1997	6.88
1998	7.03
1999	7.19
2000	7.34
2001	7.5
2002	7.66
2003	7.81
2004	7.97
2005	8.14
2006	8.3
2007	8.46
2008	8.63
2009	8.8
2010	8.96
2011	9.13
2012	9.3
2013	9.47
2014	9.64
2015	9.81