Year	Years
1970	3.42
1971	3.51
1972	3.61
1973	3.71
1974	3.81
1975	3.91
1976	4.01
1977	4.12
1978	4.23
1979	4.34
1980	4.45
1981	4.56
1982	4.68
1983	4.79
1984	4.91
1985	5.03
1986	5.16
1987	5.28
1988	5.4
1989	5.53
1990	5.66
1991	5.79
1992	5.92
1993	6.05
1994	6.19
1995	6.32
1996	6.46
1997	6.6
1998	6.74
1999	6.88
2000	7.02
2001	7.17
2002	7.32
2003	7.47
2004	7.62
2005	7.78
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	9.24
2015	9.4