Year	Years
1970	4.26
1971	4.35
1972	4.44
1973	4.54
1974	4.64
1975	4.73
1976	4.83
1977	4.93
1978	5.03
1979	5.14
1980	5.24
1981	5.34
1982	5.45
1983	5.56
1984	5.66
1985	5.77
1986	5.88
1987	5.99
1988	6.1
1989	6.21
1990	6.33
1991	6.44
1992	6.56
1993	6.67
1994	6.79
1995	6.9
1996	7.02
1997	7.14
1998	7.26
1999	7.39
2000	7.51
2001	7.64
2002	7.76
2003	7.89
2004	8.02
2005	8.16
2006	8.29
2007	8.42
2008	8.56
2009	8.69
2010	8.83
2011	8.96
2012	9.1
2013	9.24
2014	9.38
2015	9.52