Year	Years
1970	4.36
1971	4.46
1972	4.56
1973	4.66
1974	4.76
1975	4.86
1976	4.96
1977	5.07
1978	5.17
1979	5.28
1980	5.39
1981	5.5
1982	5.61
1983	5.73
1984	5.84
1985	5.96
1986	6.07
1987	6.19
1988	6.31
1989	6.43
1990	6.55
1991	6.67
1992	6.8
1993	6.92
1994	7.05
1995	7.17
1996	7.29
1997	7.42
1998	7.54
1999	7.67
2000	7.79
2001	7.92
2002	8.05
2003	8.18
2004	8.31
2005	8.44
2006	8.58
2007	8.71
2008	8.84
2009	8.98
2010	9.11
2011	9.25
2012	9.38
2013	9.52
2014	9.66
2015	9.79