Year	Years
1970	3.58
1971	3.66
1972	3.75
1973	3.83
1974	3.92
1975	4
1976	4.09
1977	4.18
1978	4.27
1979	4.36
1980	4.46
1981	4.55
1982	4.65
1983	4.75
1984	4.85
1985	4.95
1986	5.05
1987	5.16
1988	5.26
1989	5.37
1990	5.48
1991	5.59
1992	5.7
1993	5.81
1994	5.93
1995	6.04
1996	6.16
1997	6.28
1998	6.4
1999	6.52
2000	6.64
2001	6.77
2002	6.89
2003	7.02
2004	7.15
2005	7.28
2006	7.41
2007	7.54
2008	7.67
2009	7.8
2010	7.93
2011	8.07
2012	8.2
2013	8.34
2014	8.48
2015	8.61