Year	Years
1970	3.48
1971	3.56
1972	3.65
1973	3.74
1974	3.83
1975	3.92
1976	4.01
1977	4.1
1978	4.2
1979	4.3
1980	4.4
1981	4.5
1982	4.6
1983	4.7
1984	4.81
1985	4.92
1986	5.03
1987	5.14
1988	5.25
1989	5.37
1990	5.49
1991	5.6
1992	5.72
1993	5.84
1994	5.97
1995	6.09
1996	6.22
1997	6.34
1998	6.47
1999	6.6
2000	6.73
2001	6.86
2002	6.99
2003	7.12
2004	7.25
2005	7.38
2006	7.51
2007	7.64
2008	7.77
2009	7.9
2010	8.04
2011	8.17
2012	8.3
2013	8.43
2014	8.57
2015	8.7