Year	Years
1970	3.97
1971	4.07
1972	4.17
1973	4.28
1974	4.39
1975	4.5
1976	4.61
1977	4.72
1978	4.84
1979	4.96
1980	5.08
1981	5.2
1982	5.32
1983	5.45
1984	5.58
1985	5.71
1986	5.84
1987	5.98
1988	6.11
1989	6.26
1990	6.4
1991	6.55
1992	6.7
1993	6.86
1994	7.02
1995	7.19
1996	7.36
1997	7.54
1998	7.72
1999	7.89
2000	8.07
2001	8.24
2002	8.42
2003	8.59
2004	8.77
2005	8.94
2006	9.12
2007	9.29
2008	9.46
2009	9.64
2010	9.81
2011	9.98
2012	10.15
2013	10.32
2014	10.49
2015	10.66