Year	Years
1970	4.66
1971	4.77
1972	4.88
1973	4.99
1974	5.1
1975	5.21
1976	5.32
1977	5.43
1978	5.54
1979	5.65
1980	5.76
1981	5.87
1982	5.98
1983	6.09
1984	6.2
1985	6.3
1986	6.41
1987	6.52
1988	6.62
1989	6.72
1990	6.82
1991	6.92
1992	7.02
1993	7.11
1994	7.21
1995	7.3
1996	7.42
1997	7.54
1998	7.66
1999	7.79
2000	7.93
2001	8.06
2002	8.2
2003	8.34
2004	8.49
2005	8.63
2006	8.78
2007	8.93
2008	9.09
2009	9.24
2010	9.4
2011	9.56
2012	9.72
2013	9.88
2014	10.04
2015	10.2