Year	Years
1970	3.33
1971	3.41
1972	3.49
1973	3.57
1974	3.66
1975	3.75
1976	3.83
1977	3.92
1978	4.02
1979	4.11
1980	4.2
1981	4.3
1982	4.4
1983	4.5
1984	4.6
1985	4.71
1986	4.81
1987	4.92
1988	5.03
1989	5.14
1990	5.25
1991	5.36
1992	5.48
1993	5.6
1994	5.72
1995	5.84
1996	5.96
1997	6.09
1998	6.21
1999	6.34
2000	6.47
2001	6.59
2002	6.72
2003	6.85
2004	6.98
2005	7.12
2006	7.25
2007	7.38
2008	7.51
2009	7.65
2010	7.78
2011	7.92
2012	8.06
2013	8.19
2014	8.33
2015	8.46