Year	Years
1970	4.47
1971	4.57
1972	4.67
1973	4.77
1974	4.87
1975	4.98
1976	5.08
1977	5.19
1978	5.3
1979	5.41
1980	5.52
1981	5.63
1982	5.75
1983	5.86
1984	5.98
1985	6.09
1986	6.21
1987	6.33
1988	6.45
1989	6.57
1990	6.69
1991	6.82
1992	6.94
1993	7.07
1994	7.19
1995	7.32
1996	7.44
1997	7.57
1998	7.7
1999	7.83
2000	7.96
2001	8.09
2002	8.23
2003	8.36
2004	8.49
2005	8.63
2006	8.76
2007	8.9
2008	9.03
2009	9.17
2010	9.31
2011	9.44
2012	9.58
2013	9.72
2014	9.86
2015	9.99