Year	Years
1970	3.5
1971	3.58
1972	3.67
1973	3.75
1974	3.84
1975	3.93
1976	4.02
1977	4.11
1978	4.2
1979	4.3
1980	4.39
1981	4.49
1982	4.59
1983	4.7
1984	4.8
1985	4.91
1986	5.02
1987	5.13
1988	5.24
1989	5.35
1990	5.47
1991	5.59
1992	5.71
1993	5.84
1994	5.96
1995	6.09
1996	6.22
1997	6.36
1998	6.5
1999	6.63
2000	6.77
2001	6.91
2002	7.05
2003	7.19
2004	7.33
2005	7.47
2006	7.61
2007	7.75
2008	7.89
2009	8.04
2010	8.18
2011	8.33
2012	8.47
2013	8.62
2014	8.76
2015	8.91