Year	Years
1970	3.44
1971	3.52
1972	3.61
1973	3.69
1974	3.78
1975	3.87
1976	3.96
1977	4.05
1978	4.15
1979	4.25
1980	4.34
1981	4.44
1982	4.54
1983	4.65
1984	4.75
1985	4.86
1986	4.97
1987	5.08
1988	5.19
1989	5.3
1990	5.42
1991	5.53
1992	5.65
1993	5.77
1994	5.89
1995	6.02
1996	6.14
1997	6.26
1998	6.38
1999	6.51
2000	6.63
2001	6.76
2002	6.89
2003	7.02
2004	7.15
2005	7.28
2006	7.41
2007	7.54
2008	7.67
2009	7.81
2010	7.94
2011	8.08
2012	8.21
2013	8.35
2014	8.49
2015	8.62