Year	Years
1970	3.77
1971	3.86
1972	3.94
1973	4.04
1974	4.13
1975	4.22
1976	4.32
1977	4.42
1978	4.51
1979	4.62
1980	4.72
1981	4.82
1982	4.93
1983	5.04
1984	5.15
1985	5.26
1986	5.37
1987	5.48
1988	5.6
1989	5.72
1990	5.84
1991	5.96
1992	6.09
1993	6.21
1994	6.34
1995	6.47
1996	6.6
1997	6.73
1998	6.86
1999	6.99
2000	7.12
2001	7.25
2002	7.39
2003	7.52
2004	7.65
2005	7.79
2006	7.92
2007	8.06
2008	8.2
2009	8.33
2010	8.47
2011	8.61
2012	8.75
2013	8.89
2014	9.03
2015	9.16