Year	Years
1970	4.52
1971	4.61
1972	4.71
1973	4.82
1974	4.92
1975	5.02
1976	5.13
1977	5.23
1978	5.34
1979	5.45
1980	5.55
1981	5.66
1982	5.77
1983	5.88
1984	5.99
1985	6.11
1986	6.22
1987	6.33
1988	6.44
1989	6.55
1990	6.67
1991	6.78
1992	6.89
1993	7
1994	7.11
1995	7.23
1996	7.34
1997	7.45
1998	7.56
1999	7.68
2000	7.79
2001	7.91
2002	8.03
2003	8.15
2004	8.27
2005	8.39
2006	8.51
2007	8.64
2008	8.77
2009	8.89
2010	9.02
2011	9.16
2012	9.29
2013	9.42
2014	9.56
2015	9.69