Year	Years
1970	3.27
1971	3.36
1972	3.45
1973	3.54
1974	3.64
1975	3.73
1976	3.83
1977	3.93
1978	4.04
1979	4.15
1980	4.26
1981	4.37
1982	4.49
1983	4.61
1984	4.73
1985	4.85
1986	4.98
1987	5.11
1988	5.25
1989	5.39
1990	5.53
1991	5.67
1992	5.82
1993	5.96
1994	6.11
1995	6.26
1996	6.41
1997	6.55
1998	6.69
1999	6.83
2000	6.98
2001	7.12
2002	7.26
2003	7.4
2004	7.54
2005	7.69
2006	7.83
2007	7.97
2008	8.11
2009	8.25
2010	8.39
2011	8.53
2012	8.67
2013	8.81
2014	8.95
2015	9.09