Year	Years
1970	3.28
1971	3.39
1972	3.49
1973	3.6
1974	3.72
1975	3.83
1976	3.96
1977	4.08
1978	4.21
1979	4.35
1980	4.48
1981	4.63
1982	4.77
1983	4.93
1984	5.08
1985	5.24
1986	5.41
1987	5.58
1988	5.75
1989	5.93
1990	6.1
1991	6.29
1992	6.47
1993	6.65
1994	6.84
1995	7.02
1996	7.16
1997	7.3
1998	7.43
1999	7.57
2000	7.7
2001	7.84
2002	7.98
2003	8.11
2004	8.24
2005	8.38
2006	8.51
2007	8.65
2008	8.78
2009	8.92
2010	9.05
2011	9.19
2012	9.33
2013	9.46
2014	9.6
2015	9.73