Year	Years
1970	4.32
1971	4.41
1972	4.5
1973	4.59
1974	4.69
1975	4.78
1976	4.88
1977	4.97
1978	5.07
1979	5.16
1980	5.26
1981	5.35
1982	5.45
1983	5.55
1984	5.64
1985	5.74
1986	5.83
1987	5.93
1988	6.02
1989	6.12
1990	6.21
1991	6.3
1992	6.39
1993	6.48
1994	6.56
1995	6.65
1996	6.73
1997	6.81
1998	6.9
1999	7
2000	7.09
2001	7.19
2002	7.3
2003	7.4
2004	7.51
2005	7.62
2006	7.73
2007	7.84
2008	7.95
2009	8.07
2010	8.19
2011	8.3
2012	8.42
2013	8.55
2014	8.67
2015	8.79