Year	Years
1970	3.67
1971	3.77
1972	3.87
1973	3.98
1974	4.09
1975	4.2
1976	4.31
1977	4.42
1978	4.54
1979	4.66
1980	4.78
1981	4.9
1982	5.02
1983	5.15
1984	5.28
1985	5.41
1986	5.54
1987	5.68
1988	5.81
1989	5.95
1990	6.09
1991	6.23
1992	6.38
1993	6.52
1994	6.67
1995	6.82
1996	6.97
1997	7.13
1998	7.29
1999	7.44
2000	7.6
2001	7.76
2002	7.92
2003	8.09
2004	8.25
2005	8.42
2006	8.58
2007	8.75
2008	8.92
2009	9.09
2010	9.26
2011	9.43
2012	9.6
2013	9.77
2014	9.94
2015	10.11