Year	Years
1970	3.21
1971	3.3
1972	3.4
1973	3.49
1974	3.59
1975	3.69
1976	3.8
1977	3.9
1978	4.01
1979	4.12
1980	4.23
1981	4.34
1982	4.46
1983	4.58
1984	4.7
1985	4.82
1986	4.94
1987	5.07
1988	5.2
1989	5.33
1990	5.46
1991	5.6
1992	5.73
1993	5.87
1994	6.01
1995	6.16
1996	6.3
1997	6.45
1998	6.61
1999	6.76
2000	6.91
2001	7.07
2002	7.23
2003	7.39
2004	7.55
2005	7.71
2006	7.87
2007	8.04
2008	8.2
2009	8.37
2010	8.54
2011	8.71
2012	8.87
2013	9.04
2014	9.21
2015	9.38