Year	Years
1970	4.05
1971	4.16
1972	4.28
1973	4.39
1974	4.51
1975	4.62
1976	4.74
1977	4.87
1978	4.99
1979	5.12
1980	5.25
1981	5.38
1982	5.51
1983	5.65
1984	5.78
1985	5.92
1986	6.06
1987	6.2
1988	6.35
1989	6.49
1990	6.64
1991	6.79
1992	6.93
1993	7.09
1994	7.24
1995	7.39
1996	7.54
1997	7.7
1998	7.85
1999	8.01
2000	8.17
2001	8.33
2002	8.49
2003	8.65
2004	8.82
2005	8.98
2006	9.15
2007	9.32
2008	9.48
2009	9.65
2010	9.82
2011	9.99
2012	10.16
2013	10.33
2014	10.5
2015	10.67