Year	Years
1970	3.67
1971	3.78
1972	3.89
1973	4
1974	4.12
1975	4.23
1976	4.35
1977	4.48
1978	4.6
1979	4.73
1980	4.86
1981	5
1982	5.13
1983	5.27
1984	5.41
1985	5.56
1986	5.71
1987	5.85
1988	6.01
1989	6.16
1990	6.31
1991	6.47
1992	6.63
1993	6.79
1994	6.95
1995	7.12
1996	7.27
1997	7.43
1998	7.59
1999	7.75
2000	7.91
2001	8.07
2002	8.23
2003	8.4
2004	8.56
2005	8.72
2006	8.89
2007	9.05
2008	9.22
2009	9.39
2010	9.55
2011	9.72
2012	9.89
2013	10.05
2014	10.22
2015	10.39