Year	Years
1970	3.86
1971	3.95
1972	4.04
1973	4.13
1974	4.22
1975	4.32
1976	4.42
1977	4.51
1978	4.61
1979	4.72
1980	4.82
1981	4.92
1982	5.03
1983	5.14
1984	5.25
1985	5.36
1986	5.47
1987	5.58
1988	5.7
1989	5.81
1990	5.93
1991	6.05
1992	6.17
1993	6.29
1994	6.41
1995	6.54
1996	6.66
1997	6.79
1998	6.91
1999	7.04
2000	7.17
2001	7.3
2002	7.42
2003	7.56
2004	7.69
2005	7.82
2006	7.95
2007	8.09
2008	8.22
2009	8.36
2010	8.49
2011	8.63
2012	8.77
2013	8.91
2014	9.05
2015	9.18