Year	Years
1970	4.03
1971	4.13
1972	4.24
1973	4.34
1974	4.45
1975	4.56
1976	4.67
1977	4.79
1978	4.9
1979	5.01
1980	5.13
1981	5.24
1982	5.36
1983	5.48
1984	5.6
1985	5.71
1986	5.83
1987	5.95
1988	6.06
1989	6.18
1990	6.3
1991	6.41
1992	6.52
1993	6.64
1994	6.75
1995	6.86
1996	6.98
1997	7.1
1998	7.23
1999	7.35
2000	7.48
2001	7.62
2002	7.75
2003	7.89
2004	8.03
2005	8.17
2006	8.32
2007	8.46
2008	8.61
2009	8.76
2010	8.91
2011	9.06
2012	9.22
2013	9.37
2014	9.53
2015	9.69