Year	Years
1970	3.54
1971	3.64
1972	3.75
1973	3.86
1974	3.98
1975	4.09
1976	4.21
1977	4.33
1978	4.46
1979	4.58
1980	4.71
1981	4.84
1982	4.98
1983	5.11
1984	5.25
1985	5.39
1986	5.53
1987	5.68
1988	5.82
1989	5.97
1990	6.12
1991	6.27
1992	6.43
1993	6.58
1994	6.73
1995	6.89
1996	7.02
1997	7.16
1998	7.31
1999	7.45
2000	7.59
2001	7.74
2002	7.89
2003	8.04
2004	8.19
2005	8.35
2006	8.5
2007	8.66
2008	8.82
2009	8.98
2010	9.14
2011	9.3
2012	9.47
2013	9.63
2014	9.8
2015	9.97