Year	Years
1970	4.17
1971	4.27
1972	4.36
1973	4.46
1974	4.56
1975	4.66
1976	4.76
1977	4.87
1978	4.98
1979	5.09
1980	5.2
1981	5.31
1982	5.42
1983	5.54
1984	5.66
1985	5.78
1986	5.9
1987	6.03
1988	6.16
1989	6.29
1990	6.42
1991	6.55
1992	6.69
1993	6.83
1994	6.97
1995	7.11
1996	7.26
1997	7.41
1998	7.55
1999	7.7
2000	7.85
2001	8
2002	8.14
2003	8.29
2004	8.44
2005	8.59
2006	8.73
2007	8.88
2008	9.03
2009	9.18
2010	9.32
2011	9.47
2012	9.62
2013	9.76
2014	9.91
2015	10.05