Year	Years
1970	3.45
1971	3.55
1972	3.64
1973	3.74
1974	3.84
1975	3.94
1976	4.05
1977	4.15
1978	4.26
1979	4.37
1980	4.48
1981	4.59
1982	4.71
1983	4.82
1984	4.94
1985	5.06
1986	5.18
1987	5.3
1988	5.42
1989	5.54
1990	5.66
1991	5.79
1992	5.91
1993	6.04
1994	6.16
1995	6.29
1996	6.43
1997	6.57
1998	6.71
1999	6.86
2000	7.01
2001	7.16
2002	7.31
2003	7.46
2004	7.62
2005	7.77
2006	7.93
2007	8.09
2008	8.25
2009	8.41
2010	8.58
2011	8.74
2012	8.9
2013	9.07
2014	9.23
2015	9.4