Year	Years
1970	4.33
1971	4.43
1972	4.53
1973	4.63
1974	4.74
1975	4.84
1976	4.95
1977	5.06
1978	5.17
1979	5.28
1980	5.39
1981	5.51
1982	5.63
1983	5.75
1984	5.87
1985	5.99
1986	6.11
1987	6.24
1988	6.37
1989	6.5
1990	6.63
1991	6.76
1992	6.89
1993	7.03
1994	7.17
1995	7.31
1996	7.45
1997	7.59
1998	7.73
1999	7.87
2000	8.01
2001	8.16
2002	8.3
2003	8.44
2004	8.59
2005	8.73
2006	8.87
2007	9.02
2008	9.16
2009	9.3
2010	9.45
2011	9.59
2012	9.73
2013	9.87
2014	10.02
2015	10.16