Year	Years
1970	3.99
1971	4.1
1972	4.21
1973	4.32
1974	4.44
1975	4.56
1976	4.67
1977	4.8
1978	4.92
1979	5.05
1980	5.17
1981	5.31
1982	5.44
1983	5.57
1984	5.71
1985	5.85
1986	5.99
1987	6.13
1988	6.28
1989	6.42
1990	6.57
1991	6.72
1992	6.87
1993	7.03
1994	7.18
1995	7.34
1996	7.49
1997	7.65
1998	7.81
1999	7.97
2000	8.13
2001	8.3
2002	8.46
2003	8.62
2004	8.79
2005	8.95
2006	9.12
2007	9.29
2008	9.46
2009	9.62
2010	9.79
2011	9.96
2012	10.13
2013	10.3
2014	10.46
2015	10.63