Year	Years
1970	3.51
1971	3.61
1972	3.71
1973	3.82
1974	3.93
1975	4.04
1976	4.16
1977	4.27
1978	4.39
1979	4.52
1980	4.65
1981	4.78
1982	4.91
1983	5.05
1984	5.19
1985	5.34
1986	5.49
1987	5.64
1988	5.8
1989	5.96
1990	6.12
1991	6.29
1992	6.47
1993	6.64
1994	6.82
1995	7
1996	7.13
1997	7.25
1998	7.38
1999	7.51
2000	7.63
2001	7.75
2002	7.88
2003	8
2004	8.13
2005	8.25
2006	8.38
2007	8.51
2008	8.63
2009	8.76
2010	8.89
2011	9.02
2012	9.15
2013	9.28
2014	9.41
2015	9.54