Year	Years
1970	3.53
1971	3.61
1972	3.7
1973	3.79
1974	3.89
1975	3.98
1976	4.08
1977	4.18
1978	4.29
1979	4.39
1980	4.5
1981	4.61
1982	4.72
1983	4.84
1984	4.96
1985	5.08
1986	5.2
1987	5.33
1988	5.46
1989	5.6
1990	5.73
1991	5.87
1992	6.02
1993	6.17
1994	6.32
1995	6.47
1996	6.62
1997	6.78
1998	6.93
1999	7.08
2000	7.23
2001	7.39
2002	7.54
2003	7.69
2004	7.84
2005	7.99
2006	8.14
2007	8.29
2008	8.44
2009	8.59
2010	8.74
2011	8.89
2012	9.03
2013	9.18
2014	9.32
2015	9.47