Year	Years
1970	3.43
1971	3.52
1972	3.6
1973	3.68
1974	3.77
1975	3.86
1976	3.95
1977	4.04
1978	4.13
1979	4.22
1980	4.32
1981	4.42
1982	4.52
1983	4.62
1984	4.72
1985	4.82
1986	4.93
1987	5.03
1988	5.14
1989	5.25
1990	5.36
1991	5.47
1992	5.59
1993	5.7
1994	5.82
1995	5.94
1996	6.06
1997	6.18
1998	6.3
1999	6.42
2000	6.54
2001	6.67
2002	6.79
2003	6.92
2004	7.05
2005	7.17
2006	7.3
2007	7.43
2008	7.57
2009	7.7
2010	7.83
2011	7.97
2012	8.1
2013	8.24
2014	8.37
2015	8.51