Year	Years
1970	3.36
1971	3.44
1972	3.52
1973	3.61
1974	3.69
1975	3.77
1976	3.86
1977	3.95
1978	4.04
1979	4.13
1980	4.22
1981	4.32
1982	4.41
1983	4.51
1984	4.61
1985	4.71
1986	4.81
1987	4.91
1988	5.02
1989	5.12
1990	5.23
1991	5.34
1992	5.45
1993	5.56
1994	5.68
1995	5.79
1996	5.91
1997	6.03
1998	6.14
1999	6.26
2000	6.39
2001	6.51
2002	6.63
2003	6.76
2004	6.88
2005	7.01
2006	7.14
2007	7.27
2008	7.4
2009	7.53
2010	7.66
2011	7.8
2012	7.93
2013	8.06
2014	8.2
2015	8.34