Year	Years
1970	4.02
1971	4.11
1972	4.2
1973	4.29
1974	4.39
1975	4.48
1976	4.58
1977	4.68
1978	4.78
1979	4.88
1980	4.98
1981	5.08
1982	5.19
1983	5.3
1984	5.4
1985	5.51
1986	5.63
1987	5.74
1988	5.85
1989	5.97
1990	6.09
1991	6.21
1992	6.33
1993	6.45
1994	6.58
1995	6.7
1996	6.84
1997	6.97
1998	7.11
1999	7.24
2000	7.38
2001	7.52
2002	7.65
2003	7.79
2004	7.94
2005	8.08
2006	8.22
2007	8.36
2008	8.51
2009	8.65
2010	8.8
2011	8.94
2012	9.09
2013	9.23
2014	9.38
2015	9.53