Year	Years
1970	4
1971	4.09
1972	4.19
1973	4.29
1974	4.39
1975	4.49
1976	4.6
1977	4.7
1978	4.8
1979	4.91
1980	5.01
1981	5.12
1982	5.23
1983	5.33
1984	5.44
1985	5.54
1986	5.65
1987	5.76
1988	5.86
1989	5.97
1990	6.07
1991	6.18
1992	6.28
1993	6.39
1994	6.49
1995	6.6
1996	6.73
1997	6.87
1998	7
1999	7.15
2000	7.29
2001	7.44
2002	7.58
2003	7.74
2004	7.89
2005	8.04
2006	8.2
2007	8.36
2008	8.52
2009	8.68
2010	8.84
2011	9
2012	9.16
2013	9.33
2014	9.49
2015	9.66