Year	Years
1970	4.17
1971	4.27
1972	4.38
1973	4.49
1974	4.6
1975	4.71
1976	4.82
1977	4.94
1978	5.05
1979	5.17
1980	5.28
1981	5.4
1982	5.52
1983	5.64
1984	5.76
1985	5.88
1986	6
1987	6.12
1988	6.24
1989	6.36
1990	6.48
1991	6.6
1992	6.73
1993	6.85
1994	6.97
1995	7.09
1996	7.23
1997	7.37
1998	7.52
1999	7.67
2000	7.82
2001	7.97
2002	8.13
2003	8.28
2004	8.44
2005	8.6
2006	8.76
2007	8.92
2008	9.08
2009	9.25
2010	9.41
2011	9.58
2012	9.74
2013	9.91
2014	10.07
2015	10.24