Year	Years
1970	3.93
1971	4.04
1972	4.15
1973	4.26
1974	4.38
1975	4.49
1976	4.61
1977	4.73
1978	4.85
1979	4.98
1980	5.1
1981	5.23
1982	5.35
1983	5.48
1984	5.61
1985	5.74
1986	5.88
1987	6.01
1988	6.15
1989	6.28
1990	6.42
1991	6.56
1992	6.69
1993	6.83
1994	6.97
1995	7.11
1996	7.25
1997	7.39
1998	7.54
1999	7.69
2000	7.83
2001	7.98
2002	8.14
2003	8.29
2004	8.44
2005	8.6
2006	8.76
2007	8.91
2008	9.07
2009	9.23
2010	9.4
2011	9.56
2012	9.72
2013	9.88
2014	10.05
2015	10.21