Year	Years
1970	3.36
1971	3.44
1972	3.52
1973	3.6
1974	3.69
1975	3.77
1976	3.86
1977	3.94
1978	4.03
1979	4.12
1980	4.21
1981	4.31
1982	4.4
1983	4.5
1984	4.6
1985	4.7
1986	4.8
1987	4.9
1988	5
1989	5.11
1990	5.21
1991	5.32
1992	5.43
1993	5.54
1994	5.65
1995	5.77
1996	5.88
1997	5.99
1998	6.11
1999	6.23
2000	6.35
2001	6.47
2002	6.59
2003	6.72
2004	6.84
2005	6.97
2006	7.09
2007	7.22
2008	7.35
2009	7.48
2010	7.61
2011	7.74
2012	7.88
2013	8.01
2014	8.15
2015	8.28