Year	Years
1970	4.17
1971	4.27
1972	4.37
1973	4.47
1974	4.57
1975	4.67
1976	4.78
1977	4.89
1978	5
1979	5.11
1980	5.22
1981	5.34
1982	5.45
1983	5.57
1984	5.69
1985	5.82
1986	5.94
1987	6.07
1988	6.19
1989	6.32
1990	6.45
1991	6.58
1992	6.71
1993	6.85
1994	6.98
1995	7.12
1996	7.25
1997	7.38
1998	7.51
1999	7.64
2000	7.78
2001	7.91
2002	8.04
2003	8.18
2004	8.31
2005	8.45
2006	8.59
2007	8.72
2008	8.86
2009	9
2010	9.13
2011	9.27
2012	9.41
2013	9.55
2014	9.69
2015	9.83