Year	Years
1970	3.04
1971	3.12
1972	3.21
1973	3.3
1974	3.4
1975	3.49
1976	3.59
1977	3.69
1978	3.79
1979	3.89
1980	4
1981	4.11
1982	4.22
1983	4.33
1984	4.44
1985	4.56
1986	4.68
1987	4.8
1988	4.92
1989	5.05
1990	5.17
1991	5.3
1992	5.43
1993	5.57
1994	5.7
1995	5.84
1996	5.98
1997	6.13
1998	6.27
1999	6.42
2000	6.57
2001	6.73
2002	6.88
2003	7.04
2004	7.19
2005	7.35
2006	7.51
2007	7.67
2008	7.84
2009	8
2010	8.17
2011	8.33
2012	8.5
2013	8.67
2014	8.84
2015	9.01